Add inactivity timer to turn off keyboard backlight

- Should turn off backlight after inactivity of 30s
- Should turn on backlight if user does any activity
- If Display is turned off backlight should be turned off
- If Display is turned on backlight should also be turned on
- Can't rely completely on Display turn on/off logic since, apps
can acquire wakelock to keep Display on (e.g. watching a video),
but this should not stop the keyboard backlight from turning off.

Test: atest KeyboardBacklightControllerTests
Bug: 245506418
Change-Id: I024a9c507ee42e42896c501e9bb1cb83ca315e69