Delegate HFP Audio focus (HF) to entity controlling the actual phone call.

HF role currently handles audio focus itself although the idea is to
have telecom stack do it (it already does in ConnectionService and
CallAudioManager). We rip out the Audio focus management and ringer
control from the state machine and only leave the audio routing and
volume control here.

SCO routing and volume control is currently still handled by Hfp Client
since audio manager does not yet support HF responsibilities.

Bug: b/25261431

Change-Id: I893654de2427cf031c7cddda888ed3694b0001be