CEC: Prevent overwriting volume control Settings

Prevent overwriting CEC volume control Settings value when disabling CEC
globally.

This prevents an issue where disabling and re-enabling CEC will leave
volume control disabled, as the Settings value gets set to false when
disabling CEC.

Bug: 158307190
Test: atest HdmiControlService
Change-Id: I34d24185f284dfb89c02f39a3c456045d1ddd227