Fixing issue with double tap sometimes launching the same task.

- When double tapping, the next preload call can come in before activity
  start, which then resets the instance load plan (which is not 
  preloaded since Recents is “on top”), thus having no launch task, 
  which then falls back to launching the front-most task.  The preload
  call should never create a new plan unless it will preload it.

Change-Id: I001bfc901856c4e6b4e4b9b6a2125085b53c808b