Add KeyboardBacklightListener APIs to listen to backlight changes

API is going to be used by SysUI to show a popup UI, on user
triggered backlight changes.
Currently don't have ambient light based backlight changes
supported but will be supported in Android U. SysUI pop up
will be shown only when user triggered.

Bug: 261570986
Test: atest KeyboardBacklightControlletTests
Test: atest KeyboardBacklightListenerTest
Change-Id: Iba74f10b903744081196e1f9b7c89ad606dc15f1