Adjust volume bar visibility in HDMI-CEC system audio mode

When HDMI-CEC system audio mode is activated.
1. Hide volume bar when volume button is pressed in TV
2. Show volume bar when TV receives volume notification from
   Audio Receiver.

Otherwise, (system audio mode off) follows normal TV's behavior.

Bug: 17347499

Change-Id: I1f5bc14285d60d8626a8fbbef9e1959cae7d193b