Wait for TaskFragmentOrganizer to finish handling transaction

When a TaskFragmentTransaction is sent to the organizer during a
transition, have the transition wait for the organizer to finish
handling the transaction.

Bug: 207070762
Test: atest WmTests:TaskFragmentOrganizerControllerTest
Change-Id: I4b7955171cd8ce386686ff2cd64b7c04a6436ddf