Fixing crash loop when the parent affiliated task is removed.

- Instead, since we get the tasks in order, we use the previous task
  lastActiveTime as the base point for the entire affiliated task group
  (which gets brought to the front together always) if the parent
  task is not available.

Bug: 26896738
Change-Id: Id034af3f13355442f33b22ca18f9ffffa0f9d3d5