Correctly sort PermissionAppsFragment when toggling system apps.

My previous commit accidentally modified the behavior so that showing
system apps would add them to the bottom of the list.  This restores
the old behavior of keeping things in sorted order.

Test: Show/hide system apps, see alphabetized list.
Change-Id: I1a8f65c76b92a49c7ecf427a7c8165653e21a4e4