Add API (backed by Setting) to enable/disable volume changes via HDMI CEC for HDMI CEC source devices. This state is persisted via Settings. If volume control is disabled, no incoming HDMI CEC commands related to volume will be processed. If disabled, no HDMI CEC volume control messages will be sent by the device. Test: atest com.android.server.hdmi Bug: 149800547 Change-Id: I83ae9b423122b540b9adb156fb1c6f5964dd6105