Add a single-top root task for split screen

Make direct children of a created-by-organizer task organizable.
Add a single-top root task which the split divider bar can attach to. So
we can remove the special logic of the divider bar z-ordering in the
followup patch.

Bug: 207185041
Fix: 215322621
Test: atest ZOrderingTests
Test: atest com.android.wm.shell.splitscreen
Test: manual check the divider bar z-order setup with pip window and
      assistant voice plate
Change-Id: Ia36d7529ef52685be4516b6f6ea5d7179fc26fd6