Add basic structure for keyboard backlight indicator functionality

- KeyboardBacklightInteractor produces events whenever keyboard backlight changes
- That is consumed by BacklightDialogViewModel which based on that decides when dialog should be shown or hidden
- That dialog state is consumed by KeyboardBacklightDialogCoordinator which creates and shows dialog or hides it
- BacklightDialogContentViewModel is viewModel for dialog itself

Flag: KEYBOARD_BACKLIGHT_INDICATOR
Fixes: 268645018
Fixes: 268645743
Test: KeyboardBacklightChangeInteractorTest
Test: BacklightDialogViewModelTest
Change-Id: If2029ad8107e018e822237b576578444c34b9212