AudioPlaybackConfiguration has a player control interface

An AudioPlaybackConfiguration contains an IPlayer
  interface for system control of a player. It is not
  exposed to non-system signature components.
AudioService, through PlaybackActivityMonitor, is monitoring
  the death of the IPlayer so the matching player can get
  unregistered  in case it meets its maker.

Test: use vendor/google_toolbox/team/audio/cmds/ClPlaybackActivity
Bug: 30258418

Change-Id: Ibf3bceba91882ff16bffbf1219c55a1f89ccb13f