Workaround for empty recents, defer the enter animation.

- When docking the first time after starting up, the enter-animation 
  callback is not dispatched in the same way, and we end up with the
  callback before the first layout.  In such cases, defer sending the
  event which triggers the animation until the next frame when the
  layout is stable.

Bug: 28705801
Change-Id: If62eb03d6f297bff06982325ada9cd7f388438c3