Change the HDMI_ARC audio output device to full volume.

Much of our logic for volume handling using ARC relies on incorrect assumptions. Alongside the switch to full volume, this CL makes the following changes to align our logic with the current capabilities of HDMI CEC.
1. We no longer attempt to track the Audio System's volume/mute state.
2. We no longer support setting the Audio System's volume to a precise
level, or setting its absolute mute state.
3. When adjusting stream volume, depending on the adjustment, we send a single CEC UI command out of "Volume Up", "Volume Down", and "Mute" (which toggles mute).

Bug: 191230338
Test: Manual with TV connected to Audio System
Change-Id: If4d5e2042bbadcec1903152c29de7c55bb0ace22