Change most of PrivateDnsConfiguration methods to use ServerIdentity

Passing DnsTlsServer might be confusing because it's not straightforward
to know if a DnsTlsServer is a copy or onwed by PrivateDnsConfiguration.

This CL changes most of the methods to use ServerIdentity. The methods
can then get the corresponding DnsTlsServer by the new added method
getPrivateDns().

Bug: 186177613
Test: cd packages/modules/DnsResolver && atest
Change-Id: Ied4a4ee026862cd2c596586499cbfa7646eaaf2a