AudioSystem: add API to query audio HW sync source

Add a method to query from the audio HAL the HW sync
source used for a given audio session.
Modify audio policy to select a direct output with HW sync
when requested.

Bug: 16132368.

Change-Id: I03038f9188f2d389f8a5fd76a671854013a4513e
diff --git a/services/audioflinger/AudioFlinger.h b/services/audioflinger/AudioFlinger.h
index 31c5a1a..4e9d49b 100644
--- a/services/audioflinger/AudioFlinger.h
+++ b/services/audioflinger/AudioFlinger.h
@@ -248,6 +248,9 @@
     /* Set audio port configuration */
     virtual status_t setAudioPortConfig(const struct audio_port_config *config);
 
+    /* Get the HW synchronization source used for an audio session */
+    virtual audio_hw_sync_t getAudioHwSyncForSession(audio_session_t sessionId);
+
     virtual     status_t    onTransact(
                                 uint32_t code,
                                 const Parcel& data,