Stop checking IP address with the DoH list if the hostname is known

If the private DNS hostname is known, the candidate DoH IP addresses
are resolved from the hostname. We already do hostname verification
to ensure the validity of the server, so we don't need to strictly
limit the DoH IP addresses in such case.

Bug: 283708512
Test: set private DNS to dns.google/dns64.dns.google/cloudflare-dns.com
      and saw DoH queries were sent.
Change-Id: I359f14d46495dc55fd906ffeb624b7a2b8453acc