Refactor transport controls APIs

This merges TransportPerformer into MediaSession + a
TransportControlsCallback and makes TransportController into an
inner class on MediaController called TransportControls. Also makes
the PlaybackState and Metadata part of the session APIs instead of
transport controls.

Change-Id: I16ad392e6d318abe3119ad5d89656d253af25e16
10 files changed