Add keyboard backlight controller

This CL includes logic for:
- Handling keyboard backlight increment/decrement based on KeyEvents
- Save/Restore backlight when Keyboard is reconnected

Note: Currently, based on preliminary PRD, we are supporting 10
levels of keyboard backlight

Test: atest FrameworksServicesTests:KeyboardBacklightControllerTests
Bug: 245506418
Change-Id: Id7bffa43c1d58c3a64b2e125b178713cb51e4a63