audio policy: favor mixed over direct output for PCM format

modify getOutputForDevice() to skip direct output selection
if the request can obviously be attached to a mixed output
This prevents from selecting a direct output profile that
can manage stereo PCM format if a mixer output is available
to reach the same device.

Bug: 17628413.
Change-Id: I02e9a0b02c9ae5f44ff5768ac4ed02ce84d521c5