Fix memory leak in accessibility cache.

We were not clearing the cache if window state change event
is fired which was the behavior before adding the window
inspection APIs. As a result if no accessibility service
cares about windows we get a leek. Also when the accessibiilty
service cares about windows we were not clearing all windows
nodes from the cache. Now if a windows change or a window
state change event is recived we clear the cache.

bug:17589257

Change-Id: I8c416fbcab623160c6f505128b9fca713fcc6623