Like in the drm backend we use an InputEventFilter to turn the backlight
on and off. In this case the filter needs to be always installed as key
event should also turn the screen off. We cannot use a global shortcut
as it would not trigger while the screen is locked.
Unlike the filter in DRM backend only the power key can toggle the
backlight, other key events are ignored. For pointer we are optimistic
and already support enabling the backlight. Double tap to wake is also
implemented, but at least on reference hardware (Nexus 5) doesn't work
as disabling the backlight also disables touch input.
Reviewed-By: Bhushan Shah
1c4c5caf65