Decouple PrivateDnsConfiguration from resolv_cache.h

This change is to simplify writting tests for PrivateDnsConfiguration.

Also add a network ID check before starting any resolver configuration.

Bug: 79727473
Test: cd packages/modules/DnsResolver && atest
Change-Id: Icf0040c67aafe5318a61fc3d2b7158d8f0384367