Do not show AddUser if adding user is not allowed.

- when multi user is enabled, we should keep the current visibility for
AddUser preference, since earlier check has been done to update its
visibility according to the add user capability. We should only set the
visibility to false if multi user is disabled.

Change-Id: I246e9242f255dbd57c5309b2d16c95d202607531
Fixes: 114241868
Test: make RunSettingsRoboTests