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