Fix crash when deleting task

- There are several code paths from the loaders (which run on a background
  thread) which post the call to notify an update on the task which was
  loaded. Not all of these are cleared when a task is unbound, and can
  result in a notifyTaskDataLoaded() after the task is unbound. For now,
  just ensure that the TaskView is bound to the Task before updating.

Bug: 62194807
Test: Have not been able to repro, just ensure that recents thumbnails still
      load

Change-Id: Id9301025275f4b14a2832f7f6c1ebd5a1ce124ea