WindowManager: remove freezing window to fix UI freezing issue

 In some corner case, some window is at exiting state, but can't be
 removed. This will result the windownmanger fall into freezing state.
 The fix will remove this exiting window when encounter this case.

CRs-Fixed: 803491
Change-Id: I8ba72ab0566e9d13a0405be1fa4472f826d0af50