Fix adding wifi configuration issue

- The Wi-Fi Enterprise Restrictions add a new
  UserManager.DISALLOW_ADD_WIFI_CONFIG restriction from T.

- When a user requests to add Wi-Fi configuration, Settings will
  reject the request if the user is not allowed.

- The above restriction need to be retrieved in advance so that the
  user's request will not be rejected.

- See the result screenshot in b/224459753#comment8

Bug: 224459753
Test: manual test
make RunSettingsRoboTests ROBOTEST_FILTER=AddAppNetworksActivity

Change-Id: I2dfb6b9946dd9dbd4eb8bf0f6549ac5cb4466f17