Synthesize DNS64 prefix in netd

Synthesize DNS64 prefix with IPv4 dns query result in the following
conditions:
1. If specify address family to IPv6 and no IPv6 addresses result is
   obtained, then if IPv4 addresses result could be obtained
2. If address family is unspecified and query results are all IPv4
   addresses

Test: built, flashed, booted
      system/netd/tests/runtests.sh passes
Bug: 78545619

Change-Id: Ia7c5963c054772f8c95b95849282e9d9d5761515