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

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