Don't return null in WS.getTopParentWindow on child detach

Fixed issue with WindowState.getTopParentWindow() returning null when
the child window is removed from it's parent. In this case it should
return itself as the method documentation states.

Change-Id: Iae40ca21241306048cae136887afc88593a6898d
Fixes: 33446267
Test: bit FrameworksServicesTests:com.android.server.wm.WindowStateTests