Abstract the general dns name server setup to avoid duplicated work.

    - add new method StartDns() in ResolverTest class to avoid duplicated
      start procedure.
    - provide the overload construct function for dns responder class with
      default parameter value.
    - add default value for SetResolversForNetwork() method.
    - define the common const variable in ResolverTest class scope.
    - add copyright information in dns_responder_client.h

Test: atest netd_unit_test netd_integration_test resolv_integration_test
Change-Id: I00812f380be7ab800d625b8cb5401445067e5a1c