[CEC Configuration] Rename getAvailableCecSettings to getUserCecSettings

Also:
* use @CecSettingName more extensively in the API and method signature.
* add getAllCecSettings() method to HdmiCecConfig.

Bug: 168020131
Test: atest HdmiCecConfigTest
Change-Id: I318357befb05324afd0132dfcbaf2d9cc6f7bf33