Refactor the setupDns to include addIpv{4|6}Dns

Move DNS server configurations and network forwardings to setupDns
together. Moving those implementation details to the same function not
only reduces code duplication, but also help us to focus on test logic.

Bug: 176507580
Test: atest
Change-Id: Ib9a9ffd303fb5a531586178d0f3de8709889d5dd