Reset mPointerIconShape properly on window manager.

Sometimes mPointerIconShape wrongly remember old icon shape,
so that it fails to specify the icon on some situation, such
as reported in the bug.
- task == null: happening tasks are overlapping and mouse moves
  directly from an edge of a task to inside of another task
- hover_exit: happening the mouse moves outside of any tasks.
  I believe I added this line but anyways it's been lost at
  some point.

Bug: 26011155
Change-Id: I0bdc345a28089bd13443d9b7b8e7c770e56247f3