Make MediaSessionCompat/MediaControllerCompat work pre 21

This adds support for cross-process communication between MediaSessionCompat
and MediaControllerCompat. This is to support apps that split their playback
service and ui into different processes and to work with the MediaRouter
support lib.

bug:18186638
Change-Id: Ib554108fd4db5a9855d482cf3d61ce3d8d9c8850
15 files changed