MediaPlayerBase API

Define as an abstract class the high level interface for
  media players. Consumers are MediaSession2, providers
  are MediaPlayer2, which will extend this class.

Bug: 64098437
Test: to be in MediaPlayer2 tests once it extends MPB
Change-Id: Id0d0fcb6d1b377a0e05a4a8e3d659e12a58fc45e