Implementation of the policing of transport control key press

events among multiple applications competing for the remote control
focus.
AudioManager defines a new API for applications to use in order to
register their BroadcastReceiver for the media button as the one
to receive the corresponding intent, but all applications at the
same time (in an ordered broadcast).
AudioService handles a stack of remote control focus owners. It
traps ACTION_MEDIA_BUTTON intents and sends a new intent to the
remote control focus owner.

Change-Id: I3c109221ecfb160cbb1ec0e40a71b241aad73812