Ensure that windows are drawn before starting transition into PiP.

- Building upon ag/2125930, we ensure that all windows are drawn before
  starting the enter PiP animation.

Bug: 37420370
Test: bit FrameworksServicesTests:com.android.server.wm.BoundsAnimationControllerTests
Test: android.server.cts.ActivityManagerPinnedStackTests

Change-Id: I73fb71681f62bbc684efedbd3d40c3e1a670db46
8 files changed