CEC: Make HDMI CEC volume control configurable

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

Merged-In: I83ae9b423122b540b9adb156fb1c6f5964dd6105
Change-Id: I83ae9b423122b540b9adb156fb1c6f5964dd6105