Fix the sorting bugs for system apps

Bug: 30770664
Test: Manual -
1. Go to settings -> Apps -> Menu -> App permissions
2. Click one permissions folder.
3. Observe the sequence of apps.
4. Click the "show system" menu.
5. Observe the sequence of apps.

Now the system apps and user apps are merged and sorted by alphabetic
order.

Change-Id: I45c933f9be30f138085f46f9515c38bf998bfb9a
(cherry picked from commit b7ea973e)