base

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