Refactor ResState to store nameserver addresses by IPSockAddr

IPSockAddr is more safer and convenient to store socket addresses,
to compare two socket addresses, and to make the code more readable.

The change also removes get_nsaddr(), a static function in res_send.cpp.

Bug: 137169582
Test: cd packages/modules/DnsResolver && atest
Change-Id: I694c293139b01a39c40cc50ba8c4f067a2ac4b07