Fix bug 5045498 Unhide Remote Control Client API

The RemoteControlClient API enables media application to have
 information such as album art, current song, playback state
 be displayed on "remote control displays", as well as be
 controlled by "remote controls".
The new API consists in:
 - a new class, RemoteControlClient
 - new methods in AudioManager to register a RemoteControlClient
 - a new class, RemoteControlClient.MetadataEditor to edit and
  apply the information associated with a RemoteControlClient,
  and which will be displayed on a "remote control display"

Change-Id: I5616085e4ad58ce0bbc1a5953b0a5a28d32f8703