Crop window input region to stack bounds.

Prevents the input region of a window from extending outside
the stack bounds. For example, if you have a non-sizeable
activity in docked mode you don't want the app getting touchs
when you tap on the side occupied by the other app.

Bug: 25710884
Change-Id: I044b4e87448fbd3eb51822e6d71e8ed8d06f55ec