CallAudioManager refactoring

Separates the handling of audio mode and audio focus into a state
machine and brings ringing, tone generation, and ringbacks under the
control of CallAudioManager.

(cherry picked from commit fc2270137059e855756769752899d613e27d12a8)

Change-Id: Idd765550c907150117b8b66f76e88868ae3785c2