Rename the *Nat64Prefix to *Nat64PrefixFromDns.

This just a rename with no functional changes at all. It is
preparation for supporting getting the NAT64 prefix from the
RA.

Bug: 150648313
Test: covered by existing ConnectivityServiceTest and Nat464XlatTest
Change-Id: Ia9a09a708870827b1e4cf068f930fa9542dd116c