Fix visibility of AddUser when toggling multiuser.

- when enabling multi user, also need to update the visibility of the
Add user preference, but need to check the add user capability as well.

Change-Id: Ia243901c7537bdb39ca3a39aed559b003f803e4d
Fixes: 115397726
Test: make RunSettingsRoboTests

src/com/android/settings/users/UserSettings.java

@@ -964,9 +964,10 @@ public class UserSettings extends SettingsPreferenceFragment
                 mAddUserWhenLockedPreferenceController.getPreferenceKey());
         mAddUserWhenLockedPreferenceController.updateState(addUserOnLockScreen);
         mMultiUserFooterPreferenceController.updateState(null /* preference */);
+        mAddUser.setVisible(mUserCaps.mCanAddUser && Utils.isDeviceProvisioned(context)
+                && mUserCaps.mUserSwitcherEnabled);
         mUserListCategory.setVisible(mUserCaps.mUserSwitcherEnabled);
         if (!mUserCaps.mUserSwitcherEnabled) {
-            mAddUser.setVisible(false);
             return;
         }