Fix HDMI CEC volume control setting being ignored

When HDMI CEC is enabled, volume controls stop working - the volume
index changes internally but actual audio output stays at 100%. This
happens because MyHdmiControlStatusChangeListenerCallback.onStatusChange()
doesn't check mHdmiCecVolumeControlEnabled when calling updateHdmiCecSinkLocked(),
and MyHdmiCecVolumeControlFeatureListener.onHdmiCecVolumeControlFeature()
sets the flag but never calls updateHdmiCecSinkLocked() to apply it.
This makes the "HDMI CEC Volume Control" toggle useless.

Fix by
- Checking mHdmiCecVolumeControlEnabled in the CEC status callback
- Calling updateHdmiCecSinkLocked() when the volume control setting changes

Change-Id: Ic610c2c62cfad79a924d359d9af207362d9d08e7