Downgrade DoT connection error message to Warning

The failures of private DNS connection are common and sometimes
expected, such as firewall blocking, opportunistic private DNS
mode. It's not a DnsResolver bug, so downgrade the logging level
to Warning.

Bug: 181269159
Test: cd packages/modules/DnsResolver && mm
Change-Id: Ic56aaebce6cfa2c73dba9320119fdc9217905624