Let root task can reparent to launch root task while launch

When task move to back, we had some mechanism to make sure it is
under task display area or home task parent for legacy split.

For stage split, we should add reparent mechanism when task bring
to front to ensure task is under launch root stage(main stage)
otherwise it will launch as full screen and split dismissed.

Fix: 198554517
Test: manual
Test: pass existing tests
Change-Id: I6ded3d3fa2acc48a8b248b20f6d34b34d5387776