Fixing crash in Recents

- The change to setTasks() was causing us to queue items to be loaded 
  before the activity had even started.  This change ensures that we
  don’t do this for the dummy-transition-only stack view, and only when
  setting stack tasks for real.

Bug: 27538945
Change-Id: I4ff0413cb2fb961a4dedcad42471293bab4f955a