Add return-to logic for onTopLauncher.

- When an on-top launcher is moved to back, this commit makes sure
  that it's moved to the bottom of the home stack, and the home
  stack is moved to back.
- This commit also sets the proper return-to type for tasks launched
  from an on-top launcher.

Bug: 29393795
Change-Id: If0cfb35d95a5bdab008a6a3809319d3909a56f19