Hook up missing volume-change with AudioManager.

Whenever new volume change is notified from system audio,
hdmi control service should delegate it to AudioManager.
Note that, it should set FLAG_HDMI_SYSTEM_AUDIO_VOLUME,
which prevents audio manager from notifying volume-
change event back to hdmi control service.

Change-Id: I6621f517a4d20226aea9159bbae6b699e2c2ffd0