msmnile: Add route from voip_rx source to A2DP sinks

Route active voip_rx streams to A2DP output from
AudioPolicyManager if SCO is not available.
SCO and A2DP share the same backend slimbus ports.
During SCO to A2DP transition, there is no route for
pending writes from voip_rx output to be consumed.
Routing event coming from AudioPolicyManager will
help in writes not being delayed.
Add mixer path for audio-playback-voip output being
routed to bt-a2dp device.

Change-Id: I293f50570788c3c43ff229a939257c1160b66c53