Fix the ordering of taskevents.

- The ordering was not correct when a task switches to a different organizer.
- TaskVanished event was being sent after the TaskAppeared event and it was
  being sent to the new organizer instead of the previous one.
- Reason for this was that dispose() assumes that the organizer which is being
  disposed-off still has its state in mOrganizerStates. But it was being
  removed from the organizer even before dispose is called.

Fix: 222719752
Test: atest WindowOrganizerTests
Change-Id: Iea6462cada57ee62ec7ed322819dff45c284fa64