Honor HDMI_CONTROL_ENABLED setting in AudioService

This change updates how AudioService is updated of changes to Hdmi Cec
status (enable/disable) and availablity.

Bug: 138956107
Bug: 138373017
Test: Android TV STB: make; flashall;Volume -/+; HDMI control enable/disable.
Change-Id: Iaf83a45206be076858e13d113ba772ed5c59ef60
Merged-In: Iaf83a45206be076858e13d113ba772ed5c59ef60