Fix nullability in onAudioInfoChanged

The callback has always returned a @NonNull value, but was never marked
as such. This change also makes nullability of the MediaController's
PlaybackInfo consistent with ag/27085155.

Also, simplify downstream nullability for implementers of
onAudioInfoChanged.

Flag: EXEMPT non-functional refactor
Test: Presubmit.
Bug: 333564788
Change-Id: I9d4799d77158742c13f04de1cec9a3d4dd997fcf