[Media ML] Add @JavaOnlyStableParcelable to media classes

For mainline module to receive parcelable classes not part of
the module (i.e. framework class) as argument,
they should be either structured parcelable or public/system api.

For the latter case, we need to mark them as @JavaOnlyStableParcelable
to ensure that they can be referenced safely.

This CL adds the annotation to AudioAttributes and MediaMetadata,
which are used by ISession, ISessionController,
and ISessionControllerCallback.

Bug: 169046322
Test: build successful
Change-Id: Id2abb9f6b31e5432a631cc11aa65eb7c356451eb