Remove display policy references while waiting for window animation to complete

- Currently, the display policy references to singleton windows are
  maintained until the deferred window removal completes after a window
  animation.  This can cause issues when recreating a window because
  we validate that these windows are singletons.  But if we know that
  the window is in the process of being removed, we can reset these
  references early to reflect the actual state.

Bug: 183947954
Test: Trigger configuration change repeatedly to recreate the nav bar
      and verify it doesn't crash w/ error about singleton windows
Change-Id: I225cd7950e1e3e9f6689662d1596b60e94ef6530