SystemUI: Refresh user list a little more eagerly

Manual cherry-pick (since merge wasn't happy) of
844c92b4

Unfortunately, we don't get a signal when user restrictions such as
for adding new users change. That means the user switcher UI could get
out of sync and showed the add user option when in fact it couldn't add one.

Also cleans up an unused method.

Bug: 18292723
Change-Id: I804f22eed626bdbfa3bd98013b55d9992112a350