Consider nullable annotations in explicit nulls

[HarrisL2]

Fixes #21629

Not sure if the tests are sufficient.

[noti0na1]

How about List<@Nullable String> from Java? I would expect an annotated type. Not sure if this is supporeted by the java parser or class reader?

[HarrisL2]

Support can be added, however, in our nullification code, we chose to not recurse on type argument for generic types coming from Java, so supporting these annotations may be in conflict with that decision:

if `C` is Java-defined, then `n(C[T]) = C[T] | Null`. That is, we don't recurse
on the type argument, and only add Null on the outside. This is because
`C` itself will be nullified, and in particular so will be usages of `C`'s type argument within C's body.
e.g. calling `get` on a `java.util.List[String]` already returns `String|Null` and not `String`, so
we don't need to write `java.util.List[String | Null]`.

Now if we have a List<@Nullable String> or List<@Nonnull String>, I believe the above argument doesn't hold anymore.