Settings: More options added to hardware key rebinding (2/2) - Slim

        In the same vein as the available Navbar actions, this adds the following new hardware key rebind options:
         * Last app: Switches to the last app
         * Power button: Turns off the screen
         * Custom app: Launches an application/activity of your choice

        Change-Id: I3d9b0e447862a40a90ded22d3952cab09c9a48ab