Fix small issue with task org state cleanup

- When the task org binder died, it previously did not remove from
  the mTaskOrganizersForWindowingMode list and handle replacements
  the same way as unregistering.  Likewise when unregistering,
  the code to handle replacements could override the existing
  state since it only checks if the previous state was not disposed
  before setting it.

  Alternatively, we can keep the list of TaskOrganizerStates for a
  windowing mode in a list. The last item in the list is the current
  task org for that windowing mode. When a task org is removed or
  its binder dies, we simply remove from that list.

Bug: 152134222
Test: atest WmTests:TaskOrganizerTests

Change-Id: I1c7fd8907c7f025da25d62b0fa939edbb789e0cb