PowerManagerService: Allow 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 setting in order to allow the user
   to choose the preferred behavior.

Change-Id: I35ac71a8274568901f962c9692788d1c682a98dd