Restore correct mouse pointer after drag and drop

This CL introduces IWindow.updatePointerIcon method.
When the drag is complete, this method is called on the window
directly under the pointer.

Same method can be used in the future to update mouse pointer
when a window appears or disappears.

Bug: 24415739
Change-Id: Ia7b0522448cb3cd754da5e24696060d3b3bf2e50