Clean up some more refs to thumbnail data

- Always return a copy of the task list to ensure that the model doesn't
  hold refs to the thumbnail data if it was loaded into the same task
- Always clear the task thumbnail data ref once the visibility of the
  task view changes to be invisible

Bug: 132309376
Test: Enter overview scroll to the end of the list, relaunch app and take
      heap dump

Change-Id: I4437fd30172a5fe2a78c111f780163a1e6bbbb54