Return the existing tasks when registering an organizer

- Existing tasks for supported windowing modes are returned when
  registering the organizer and the organizer will not receive
  onTaskAppeared() for these tasks.  For now, the shell task
  organizer will keep these tasks for reporting task appeared when
  individual components add listeners for their windowing modes.
- Fixed some tests now that we organize fullscreen tasks

Bug: 162028888
Test: atest WindowOrganizerTests
Test: atest ShellTaskOrganizerTests
Change-Id: Iedc5f58fbfa819e753a5dcea7a8eb9dee8b6843b