Use ipv4only.arpa for DNS64 detection.

We currently use DNS lokoups to ipv4.google.com to do DNS64
detection. Now that RFC 7050 has been published and
ipv4only.arpa has been published by IANA, switch to using that.

Change-Id: Ia9ebc5db77364104f5888edb53468d29dc618d34
1 file changed