Restore the Private DNS settings to default mode upon network resets.

Bug: 134918038
Test: atest FrameworksNetTests NetworkStackTests
Test: manual test
      configure private dns settings (e.g. dns.google) -> connect Wi-Fi
      -> click "Reset Wi-Fi, mobile & Bluetooth" -> verify private dns
      settings is automatic -> connect Wi-Fi -> verify Internet
      connection.

Change-Id: I13562af622d7b584d6219f74534219b7868275f9