Fix bug 3275151 Request and abandon audio focus with audio mode changes

Before the introduction of the MODE_IN_COMMUNICATION audio mode,
 "calls" were only made through telephony and the PhoneStateListener
 was used to request and abandon audio focus as calls where started
 and ended. VoIP applications do not cause phone state events to
 be broadcast, and set the audio mode to MODE_IN_COMMUNICATION
 directly. This change monitors mode changes to automatically
 grab and abandon audio focus.

Change-Id: Ie9a4193e35ba8447764fac66a988959a93c4909f