Do not update pointer icon shape for HOVER_EXIT event.

EXIT event does not have to udpate the pointer icon shape, because
when it immediately moves to another window, its HOVER_ENTER will
handle the icon shape.

Also HOVER_EXIT can happen after the HOVER_ENTER of the new window,
and in that case updating pointer icon at HOVER_EXIT will overwrite
the pointer icon for the new window.

Bug: 24415739
Change-Id: I08fc72cc69bbc3a6eef36d501d93e8e9ad36df85