Allow adding a user while still removing other users

When at the user limit, removing and adding a user causes a race
condition where the deleted user is still being removed and adding
another one fails.

This change excludes deleted users from the counting to compare
against the limit.

Also fix an ArrayIndexOutOfBounds recently introduced in AppOpsService.

Bug: 13282768
Change-Id: Ib79659e7604396583a280dbbc560b288a6d9051c