User switcher in now checks for whether the user can switch to other

users.

If a user is not allowed to switch to other users, they will only see themselves
in the switcher.

Change-Id: I0f92696e8500298bfce023ff67953d110c86edf1
Fixes:110900364
Fixes:110697332
Test: Manual in mojave in both users with and without switch restriction
(cherry picked from commit 9973fffba735fcaa4e3703b8d8fd5be4e33f6284)