This is a small refactoring toward merging
MediaNotificationHandler and PlayerNotificationManager
In detail, this CL includes following changes:
- Use MediaController to dispatch commands to sessions in
MediaSessionService, rather than media key events.
- Use MediaController to monitor changes in MediaSession's
underlying Player, rather than ForegroundServiceEventCallback.
Removed the callback interface as well.
PiperOrigin-RevId: 422049265
