getnameinfo: Don't add flag NI_NUMERICHOST for well-known prefix address

Currently, getnameinfo() will never issue a reverse DNS lookup if the first
byte of the IPv6 address is 0x00. This means it is not possible to do a
reverse DNS lookup for a NAT64 address if the NAT64 prefix is the well-known
prefix. Make this possible by treating the well-known prefix specially. This
is not needed for unicast NAT64 prefixes because they do not start with 0x00.

Bug: 78545619
Test: netd_{unit,integration}_test pass
Test: bionic-unit-tests --gtest_filter='net*' pass
Change-Id: I176d30dcf411a5ffe1eec110db99cd73b48e956f