Disable "Add network" if DISALLOW_ADD_WIFI_CONFIG

- When DISALLOW_ADD_WIFI_CONFIG is not restricted by Admin
  - Disable the preference directly

Bug: 335387874
Flag: EXEMPT bugfix
Test: Manual testing
atest -c AddWifiNetworkPreferenceTest

Change-Id: I0bb08656971107c73d7632a395a820d03723620b