Update HDMI source volume change handling

Forward volume keys to HdmiControlManager if the Android device is
a HDMI source device and if the volume behavior of the audio device
is full volume.

The output audio device will be full volume if the Android device is
a HDMI source device with a sink that supports HDMI CEC OR if the user
selected HDMI CEC as the volume control mechanism.

In case the user prefers HDMI CEC, always attempt to send CEC messages,
even if the audio sink possibly does not support CEC volume changes.

Bug: 172539380
Test: make; test manually

Change-Id: I520dbebaf5ed8f1827405c65137d039be8d816e3