Make bevy_audio optional for android_shared_stdcxx

[Shatur]

Objective

• It's a bit unexpected to require bevy_audio even when it's not used with default_platform, which enables android_shared_stdcxx.

Solution

• Treat this feature similar to std. It just enables android_shared_stdcxx on cpal inside bevy_audio.

[mockersf]

I don't remember the impact at the moment, but a shared stdcxx was needed for some things on android and we were relying on cpal to do it for us

[beicause]

I think this makes sense. I can build for Android successfully when using other audio crate without relying on bevy audio and shared stdcxx.

And after #20323, bevy audio no longer needs it anyway.

[mockersf]

I can build for Android successfully when using other audio crate without relying on bevy audio and shared stdcxx.

As it's a shared library, I think testing needs to be actually playing sound on Android, not just building

And after #20323, bevy audio no longer needs it anyway.

Nice!

[Shatur]

As it's a shared library, I think testing needs to be actually playing sound on Android, not just building

But the whole point was to build for Android without requiring audio. And if it's enabled - enable android_shared_stdcxx on cpal.