audio policy: fix HW A/V sync rerouting

Do not allow HW A/V sync tracks to be attached
to a PCM mixer as those tracks carry time stamps in band.

Cherry pick of c576f6a0 from lmp-mr1-tv-dev.

Bug: 25293500.

Change-Id: I1f080232e439a410ded25ca8e461113a96687a3b
diff --git a/services/audiopolicy/managerdefault/AudioPolicyManager.cpp b/services/audiopolicy/managerdefault/AudioPolicyManager.cpp
index c3b1529..47ea8d2 100644
--- a/services/audiopolicy/managerdefault/AudioPolicyManager.cpp
+++ b/services/audiopolicy/managerdefault/AudioPolicyManager.cpp
@@ -954,6 +954,13 @@
     }
 
 non_direct_output:
+
+    // A request for HW A/V sync cannot fallback to a mixed output because time
+    // stamps are embedded in audio data
+    if ((flags & AUDIO_OUTPUT_FLAG_HW_AV_SYNC) != 0) {
+        return AUDIO_IO_HANDLE_NONE;
+    }
+
     // ignoring channel mask due to downmix capability in mixer
 
     // open a non direct output