Audio: Add CEC volume feature callback

Register a listener to HdmiControlManager to get notified when the state
of the HDMI CEC volume control feature changes.

Disabling the CEC volume control feature results in AudioService no
longer attempting to change volume via HDMI CEC.

Test: turn off CEC from Settings, verify CEC messages not sent
Bug: 152018314

Change-Id: I4be3632482fe7dc7fe8e1ad89414ea365fe459df