AVRCP: use only one MediaController from a package

Some players have more than one active MediaController for some reason,
and onActiveSessionsChanged returns them both.  In this case, the first
one in the list should be considered the "active" one for that player.

Test: use com.zentertain.music.player and see metadata propogate
Bug: 63058319
Change-Id: I4e72182b36642b84835275b0ded9648a38bde78b