Debounce messages in BrightnessController

Prevent message for register/unregister to be posted more than once.

Also, refactor BrightnessController for better testability.

Fixes: 289042855
Test: atest BrightnessControllerTest QSPanelControllerTest
Change-Id: Id72c0f2bfd6fd3720670a1a55c67526c63a043d4
6 files changed