Update task organizer state whenever task organizer was set

Root tasks that created by organizer were not added to
TaskOrganizerStates because those tasks were not visible.
In that case, these tasks were not removed when the organizer
was dead.

Bug: 153287798
Test: atest SplitScreenTests
Test: atest TaskOrganizerTests
Change-Id: Icbe1f119ae6e97f730138af83f265724bb4ae5a9