Add tests for resolv_set_nameservers_for_net in res_cache_test

This changes comprises:

[1] Convert resolv_set_nameservers_for_net to C++ containers.
[2] Unit tests for resolv_set_nameservers_for_net.
[3] Use std::vector<std::string> to store the DNS servers in struct
    resolv_cache_info.
[4] Cleanup for libnetd_resolv_test.

Bug: 134897288
Test: cd systen/netd && atest
Change-Id: I4aebd22871a4ebbed08f03391e683cabe5dc2349