Embedded Activities in TaskFragments

- Adding TaskFragments into the leaf Task to embed activities when
  activity started to the side.
- Delete TaskFragment
- Reparent TaskFragments

Bug: 189385246
Bug: 190433129
Test: wm presubmit and have test case in updating CL.
Change-Id: I7e7f5109457dd083d3b148d3ce676ad9aa5ad89f