SDK / exoplayer

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