Make GET_TASKS signature|system.

Normal apps can't hold it now.  If they try to use
getRecentTasks() or getRunningTasks() without the permission,
they will only see their own tasks and home in the list.

Also took this opportunity to eradicate all of the old pending
thumbnail stuff.

Change-Id: I6dc52a06221c78097162e4a8b482027b798bf3ee