Change ServerIdentity to store IPSockAddr instead of IPAddress

The parameter protocol was added to ServerIdentity in order to
identify an unique private DNS server. This is unneeded after
this change since we can check the socket port to achieve the
same purpose.

Bug: 186177613
Test: cd packages/modules/DnsResolver && atest
Change-Id: Ifdad55ea817ff568a84271a5d7b77dd4fbe4772c