Fix the transition from Split to PiP

- Do not bring split to top on entering PiP
  When swiping up to home (or other cases) triggers one of the
  split-screen children enters PiP, do not try to bring the other task
  to top since the pair should be considered invisible.
- When entering PiP from split screen or non-auto PiP case, use the
  alpha information in PictureInPictureSurfaceTransaction to hide the
  task which would later be turned back on to avoid flicker.

Bug: 190855091
Bug: 205894095
Video: http://recall/-/aaaaaabFQoRHlzixHdtY/42RWtayanp2qG0mHSf4Q5
Test: manual, enter PiP from split-screen, see Video
Change-Id: I1559748cd583c20d79ee458822d66e7801733d3a