Wake task persister whenever a new task is added

- The creation of a new task will trigger writing the task to disk in
  several places.  However, when a task is added to the TaskPersister
  write queue (ie. in ActivityRecord.onParentChanged), the task may
  not yet have been added to the set of RecentTasks.

  The task persister, as a part of onPreProcessItem() will try to fetch
  the persistable task ids to write from RecentTasks, which may not have
  the new task, resulting in it skipping the write of the item (because
  the set of persistable tasks have not changed).

  The task is then not persisted until the next time a new item is added
  to the write queue (can repro by starting a couple tasks, then rebooting
  and opening recents again).

  Instead, we should ensure that whenever a task is added to the recent
  task list, we poke the task persister to ensure that it will have the
  latest set of persisted tasks.

Bug: 146014211
Bug: 141560745
Test: atest RecentTasksTest
Test: Open an app, go home, open another app, wait for the task persister
      delay and verify that the tasks are in recents
      (adb shell dumpsys activity recents)
      and also that the persisted tasks have been updated with all tasks
      (adb shell cat /data/system_de/0/persisted_taskIds.txt)

Change-Id: I95f80d5fe2ef8d6c3128562bd36d4c95cd582eba