Don't copy null surface control and remove lingering references to leashes

- If a task that should otherwise be organized but does not yet have a
  surface when the task organizer is registered, then skip trying to
  return that task as we can not leash it.  Existing logic will send the
  task to the organizer once a surface control is set on it.
- In the entry points to the shell where leashes are given from WM core
  (ie. the organizers), ensure that we release them when we are notified
  that they are no longer organized

Bug: 235858985
Test: atest ShellTaskOrganizerTests
Test: atest WindowOrganizerTests
Change-Id: I16d29315b615b9756a78310d6a62fb6f21ba1184