Disabling movement of entire affiliated task set.

- This CL removes the dependency on the last-active-time of the task
  record, since that is dependent on the current system time (which can
  be changed by the user).  Instead of working around the previous
  behavior in ActivityManager which moves all affiliated tasks to the
  top when one task is launched, we change the behavior in the AM
  directly, and prevent re-sorting the list of recent tasks improperly
  (aside from the stable sort which puts the freeform tasks first).

Bug: 27398177

Change-Id: I9fa9b3497d08082fe00aa724538255de87e746d6