PowerManagerService: add a config to light up buttons only when pressed

 * Right now capactive, lit hardware keys are being
   lit every time you either touch them or the screen.

   But some devices handle this differently on stock:

   Display touch => buttons not lit
   Buttons touch => buttons lit

 * Thus, add a config in order to make this behavior
   consistent to stock ROMs for some devices.

Change-Id: I3bc214d9062f6762ebe5ab0fff949da0460a0fec