audio: Changes to enable BT A2DP

 For Proxy thread to be started , a2dp state has to updated from
 the audiopolicy.
 Now , added changes to send the BT A2DP status to audio HAL.
- A- Family doesn't support separate output for a2dp , removed the
 getA2dpOutput(). getDeviceForStrategy() is selecting the proper
 device using mA2dpSuspended flag.

CRs-Fixed: 751090
Change-Id: I599de562a8674ee611e10575fcc85e4715707a63