Reduce extraneous loading of task snapshots/icons

- Skip handling loading visible task data after resetting overview, we
  currently keep the tasks bound to their task views (some calls are not
  synchronous with the lifecycle of recents, can be changed later), so
  the next call to loadVisibleTasks before the tasks have been loaded would
  load snapshots for old tasks
- Skip loading tasks if the previously requested load plan is still valid,
  this can happen because setCurrentTask() will be called multiple times
  during swipe up

Bug: 117603579
Change-Id: Ie58ded14dedbb3934d08163ca7939d89310c3ecf