Convert to new media button receiver logic.

Commit Ide906efdf90a6f42f3fcb6c5386328301165cb7c changed the media
button receiving logic to be routed through MediaSession, but missed to
also convert the media button intent receiver registration used for
receiving button events if our playback is stopped.

Also add the possible stop action (we're handling onStop after all)
missing in the aforementioned commit.

Change-Id: Idd4b8bc00f58eee2ff7fc2e5a685b7918988c37c