Always check off-link connectivity in NetworkDiagnostics.

Currently, NetworkDiagnostics only checks off-link connectivity if
one of the DNS servers is off-link. Make it check off-link
connectivity in all cases by sending probes to Google Public DNS
if off-link DNS servers are not specified.

Bug: 22569331
Bug: 22641669
Bug: 22748900
Change-Id: I6cb0dd8491bc0c1a488631deca56722b9c1d2b3f