Bug 5567648 Request and abandon audio focus in calls

Up to now, audio focus was implicitly requested and abandoned
 when changing the audio mode. This is no longer the case so the
 behavior with regards to audio focus can be indepently set by
 the CallManager.
The logic implemented here is the same as the one previously used
 in AudioService:
  - only request audio focus when the ring volume index is > 0
    when ringing,
  - request focus before setting the audio mode to a mode other
    than normal
  - abandon audio focus after setting the audio mode to normal

Change-Id: Ia543dc779563dbff09414771fee60e589dfaab9d