Update MediaSession APIs for council feedback

-Changes MediaSessionManager.SessionsListener to OnActiveSessionsChangedListener
and makes it an interface.
-Renames MediaController.VolumeInfo to MediaController.AudioInfo
-Renames MediaSession.Track to MediaSession.Item

bug:17059552
Change-Id: I3a0b4371700ad6657972c312a5ab89ddfac6df5b
9 files changed