Only allow roots tasks to be organized

Non-root tasks are relative to their parents so no need to have them
return true to the isOrganized() method.
We can also:
- Reset the windowing mode for a task we are moving to split screen
before we reparent to the split task.
- Remove code in sys-ui that was added to compensate for the
method previous returning true.

Bug: 152619437
Test: They pass
Change-Id: I7a0381f535f2d0b245773ecb1fbb4fdb524954f9