Take task snapshot when entering PiP

- This is to ensure that we have valid thumbnail later after exit-PiP
- For multi-activity Task entering PiP case, the snapshot is taken on
  the original Task. It will otherwise be empty

Flag: EXEMPT bugfix
Bug: 433885996
Video: http://recall/-/aaaaaabFQoRHlzixHdtY/qQvRRVn7qYWYLNeBzAv5f
Video: http://recall/-/aaaaaabFQoRHlzixHdtY/duBPi2KFRDK2wkGXALGO4J
Test: Enter PiP then close, validate thumbnail in Overview, see videos
Change-Id: I1363be4e9c718b6520648b394ad9bf13aa90eea8