[Provider Model] Update Internet preference summary

- Show "Networks available" summary instead of "Not connected" when no
network connected

- Screenshot:
  https://screenshot.googleplex.com/AxhtTqpxXVSCT5o

Bug: 182244261
Test: manual test
atest -c InternetPreferenceControllerTest

Change-Id: If32f656d7d89a8b19c4da4876ccc0d1cf92262a0