audio policy: fix HW A/V sync output selection

commit b732cf5a broke the selection mechanism for the
HW A/V sync direct output.
Add a rule to only select a mixed output for basic PCM
stereo content when a direct output is not explicitly requested.

Bug: 17702382.

Change-Id: I1387690ace77cd4d6a8faf0b525c3d1008fba3f1
1 file changed