audio policy: fix set volume with explicit audio patch

setStreamVolumeIndex() receives both a stream and a device as inputs.
When an explicit audio patch is requested on an output mix, it
is possible that the active device does not match the strategy
derived from the stream, in which case the function would not update
current volume.
This patch makes sure that the device explicitly passed to
setStreamVolumeIndex() is always taken into acount.

Change-Id: Ie496ff70f059166af6c81c8f8445a9a9c7548a72