Fix SystemUI crash from continues launch task with setLaunchRootTask

In StageCoordinator#addActivityOptions, there will use
KEY_LAUNCH_ROOT_TASK_TOKEN to specify the "parent task" of current
launch, but that isn't align the definition of "root task" after
single split root was introduced. So the method "getOrCreateRootTask"
also seems weird now.
WindowContainer#reparent will throw IllegalArgumentException when no
parent change.

Bug: 286028934
Test: create a split pair, continues drag chrome to the same stage.
Change-Id: Ic165eca258a8ef1f365b4f86f98211453243a465