HFP: Suspend A2DP whenever call state is busy

* Suspend A2DP whenever call state is ringing or ongoing
* Resume A2DP when call state becomes idle and no SCO audio is
  established

Bug: 110771727
Test: Receive incoming call when in-band ringing is disabled
Change-Id: I59361384c223e64e7ce73444c4e1cdf25bf18781