Fix build breakage by updating compileSdkVersion to 28

[vishnukvmd]

The example app uses the fontVariationSettings and ttcIndex attributes which were introduced in API level 28.

Current build failure log:

example|⇒ flutter run
Launching lib/main.dart on ONEPLUS A5000 in debug mode...
[!] Your app isn't using AndroidX.
    To avoid potential build failures, you can quickly migrate your app by following the steps on
    https://goo.gl/CP92wY.
/Users/vishnu/.gradle/caches/transforms-1/files-1.1/core-1.1.0.aar/aa70ad0cbb52b70edfe2e87e61eca011/res/values/values.xml:7:5-70: AAPT: error: resource android:attr/fontVariationSettings not found.

/Users/vishnu/.gradle/caches/transforms-1/files-1.1/core-1.1.0.aar/aa70ad0cbb52b70edfe2e87e61eca011/res/values/values.xml:7:5-70: AAPT: error: resource android:attr/ttcIndex not found.

/Users/vishnu/work/android/flutter/flutter-draggable-scrollbar-master/example/build/app/intermediates/incremental/mergeDebugResources/merged.dir/values/values.xml:20: error: resource android:attr/fontVariationSettings not found.
/Users/vishnu/work/android/flutter/flutter-draggable-scrollbar-master/example/build/app/intermediates/incremental/mergeDebugResources/merged.dir/values/values.xml:20: error: resource android:attr/ttcIndex not found.
error: failed linking references.

Failed to execute aapt
com.android.ide.common.process.ProcessException: Failed to execute aapt
        at com.android.builder.core.AndroidBuilder.processResources(AndroidBuilder.java:796)
        at com.android.build.gradle.tasks.ProcessAndroidResources.invokeAaptForSplit(ProcessAndroidResources.java:551)
        at com.android.build.gradle.tasks.ProcessAndroidResources.doFullTaskAction(ProcessAndroidResources.java:285)
        at com.android.build.gradle.internal.tasks.IncrementalTask.taskAction(IncrementalTask.java:109)
        at org.gradle.internal.reflect.JavaMethod.invoke(JavaMethod.java:73)
        at org.gradle.api.internal.project.taskfactory.DefaultTaskClassInfoStore$IncrementalTaskAction.doExecute(DefaultTaskClassInfoStore.java:173)
        at org.gradle.api.internal.project.taskfactory.DefaultTaskClassInfoStore$StandardTaskAction.execute(DefaultTaskClassInfoStore.java:134)
        at org.gradle.api.internal.project.taskfactory.DefaultTaskClassInfoStore$StandardTaskAction.execute(DefaultTaskClassInfoStore.java:121)
        at org.gradle.api.internal.tasks.execution.ExecuteActionsTaskExecuter$1.run(ExecuteActionsTaskExecuter.java:122)
        at org.gradle.internal.progress.DefaultBuildOperationExecutor$RunnableBuildOperationWorker.execute(DefaultBuildOperationExecutor.java:336)
        at org.gradle.internal.progress.DefaultBuildOperationExecutor$RunnableBuildOperationWorker.execute(DefaultBuildOperationExecutor.java:328)
        at org.gradle.internal.progress.DefaultBuildOperationExecutor.execute(DefaultBuildOperationExecutor.java:197)
        at org.gradle.internal.progress.DefaultBuildOperationExecutor.run(DefaultBuildOperationExecutor.java:107)
        at org.gradle.api.internal.tasks.execution.ExecuteActionsTaskExecuter.executeAction(ExecuteActionsTaskExecuter.java:111)
        at org.gradle.api.internal.tasks.execution.ExecuteActionsTaskExecuter.executeActions(ExecuteActionsTaskExecuter.java:92)
        at org.gradle.api.internal.tasks.execution.ExecuteActionsTaskExecuter.execute(ExecuteActionsTaskExecuter.java:70)
        at org.gradle.api.internal.tasks.execution.SkipUpToDateTaskExecuter.execute(SkipUpToDateTaskExecuter.java:63)
        at org.gradle.api.internal.tasks.execution.ResolveTaskOutputCachingStateExecuter.execute(ResolveTaskOutputCachingStateExecuter.java:54)
        at org.gradle.api.internal.tasks.execution.ValidatingTaskExecuter.execute(ValidatingTaskExecuter.java:58)
        at org.gradle.api.internal.tasks.execution.SkipEmptySourceFilesTaskExecuter.execute(SkipEmptySourceFilesTaskExecuter.java:88)
        at org.gradle.api.internal.tasks.execution.ResolveTaskArtifactStateTaskExecuter.execute(ResolveTaskArtifactStateTaskExecuter.java:52)
        at org.gradle.api.internal.tasks.execution.SkipTaskWithNoActionsExecuter.execute(SkipTaskWithNoActionsExecuter.java:52)
        at org.gradle.api.internal.tasks.execution.SkipOnlyIfTaskExecuter.execute(SkipOnlyIfTaskExecuter.java:54)
        at org.gradle.api.internal.tasks.execution.ExecuteAtMostOnceTaskExecuter.execute(ExecuteAtMostOnceTaskExecuter.java:43)
        at org.gradle.api.internal.tasks.execution.CatchExceptionTaskExecuter.execute(CatchExceptionTaskExecuter.java:34)
        at org.gradle.execution.taskgraph.DefaultTaskGraphExecuter$EventFiringTaskWorker$1.run(DefaultTaskGraphExecuter.java:248)
        at org.gradle.internal.progress.DefaultBuildOperationExecutor$RunnableBuildOperationWorker.execute(DefaultBuildOperationExecutor.java:336)
        at org.gradle.internal.progress.DefaultBuildOperationExecutor$RunnableBuildOperationWorker.execute(DefaultBuildOperationExecutor.java:328)
        at org.gradle.internal.progress.DefaultBuildOperationExecutor.execute(DefaultBuildOperationExecutor.java:197)
        at org.gradle.internal.progress.DefaultBuildOperationExecutor.run(DefaultBuildOperationExecutor.java:107)
        at org.gradle.execution.taskgraph.DefaultTaskGraphExecuter$EventFiringTaskWorker.execute(DefaultTaskGraphExecuter.java:241)
        at org.gradle.execution.taskgraph.DefaultTaskGraphExecuter$EventFiringTaskWorker.execute(DefaultTaskGraphExecuter.java:230)
        at org.gradle.execution.taskgraph.DefaultTaskPlanExecutor$TaskExecutorWorker.processTask(DefaultTaskPlanExecutor.java:124)
        at org.gradle.execution.taskgraph.DefaultTaskPlanExecutor$TaskExecutorWorker.access$200(DefaultTaskPlanExecutor.java:80)
        at org.gradle.execution.taskgraph.DefaultTaskPlanExecutor$TaskExecutorWorker$1.execute(DefaultTaskPlanExecutor.java:105)
        at org.gradle.execution.taskgraph.DefaultTaskPlanExecutor$TaskExecutorWorker$1.execute(DefaultTaskPlanExecutor.java:99)
        at org.gradle.execution.taskgraph.DefaultTaskExecutionPlan.execute(DefaultTaskExecutionPlan.java:625)
        at org.gradle.execution.taskgraph.DefaultTaskExecutionPlan.executeWithTask(DefaultTaskExecutionPlan.java:580)
        at org.gradle.execution.taskgraph.DefaultTaskPlanExecutor$TaskExecutorWorker.run(DefaultTaskPlanExecutor.java:99)
        at org.gradle.internal.concurrent.ExecutorPolicy$CatchAndRecordFailures.onExecute(ExecutorPolicy.java:63)
        at org.gradle.internal.concurrent.ManagedExecutorImpl$1.run(ManagedExecutorImpl.java:46)
        at org.gradle.internal.concurrent.ThreadFactoryImpl$ManagedThreadRunnable.run(ThreadFactoryImpl.java:55)
Caused by: java.util.concurrent.ExecutionException: java.util.concurrent.ExecutionException: com.android.tools.aapt2.Aapt2Exception: AAPT2 error: check logs for details
        at com.google.common.util.concurrent.AbstractFuture.getDoneValue(AbstractFuture.java:503)
        at com.google.common.util.concurrent.AbstractFuture.get(AbstractFuture.java:482)
        at com.google.common.util.concurrent.AbstractFuture$TrustedFuture.get(AbstractFuture.java:79)
        at com.android.builder.core.AndroidBuilder.processResources(AndroidBuilder.java:794)
        ... 41 more
Caused by: java.util.concurrent.ExecutionException: com.android.tools.aapt2.Aapt2Exception: AAPT2 error: check logs for details
        at com.google.common.util.concurrent.AbstractFuture.getDoneValue(AbstractFuture.java:503)
        at com.google.common.util.concurrent.AbstractFuture.get(AbstractFuture.java:462)
        at com.google.common.util.concurrent.AbstractFuture$TrustedFuture.get(AbstractFuture.java:79)
        at com.android.builder.internal.aapt.v2.QueueableAapt2.lambda$makeValidatedPackage$1(QueueableAapt2.java:179)
Caused by: com.android.tools.aapt2.Aapt2Exception: AAPT2 error: check logs for details
        at com.android.builder.png.AaptProcess$NotifierProcessOutput.handleOutput(AaptProcess.java:463)
        at com.android.builder.png.AaptProcess$NotifierProcessOutput.err(AaptProcess.java:415)
        at com.android.builder.png.AaptProcess$ProcessOutputFacade.err(AaptProcess.java:332)
        at com.android.utils.GrabProcessOutput$1.run(GrabProcessOutput.java:104)

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':app:processDebugResources'.
> Failed to execute aapt

* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output.

* Get more help at https://help.gradle.org

BUILD FAILED in 1s
Running Gradle task 'assembleDebug'...
Running Gradle task 'assembleDebug'... Done                         2.2s
Exception: Gradle task assembleDebug failed with exit code 1

Updating the compileSdkVersion to 28 fixes this.