Don't reparent stack created by organizer.

An organizer controlls its stacks. The system should not move them to
another display.

With this CL, for stacks created by organizer, the system tries
reparenting their direct children, then removed the stacks.

Bug: 162825804
Test: atest TaskDisplayAreaTests
Change-Id: I18fcae7a8c61d83014d5ab227a5a9ab9683299a7