audio: Add audio-playback-voip bt-a2dp mixer path.

There is requirement of audio-playback-voip bt-a2dp mixer path
as , during transition phase it is possible that
APM can call a2dp routing based on setPhoneState() / setForceUse()
API.
Add audio-playback-voip bt-a2dp mixer path (to avoid out_write errors)

Change-Id: If79043b1b517948cceb9eb04b6d6f01628c315ba