More options added to hardware key rebinding (1/2)

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: I1ca98b1c615e4336c644d0247b4277b9421a96ff