Cleanup the libnetd_resolv exports

After moving DnsProxyListener into libnetd_resolv, gethostbyname and
gethostbyaddr no longer need to be publicly accessible.

However, netd is still calling android_getaddrinfofornetcontext()
directly. It should instead call android_getaddrinfofornetwork() from
multinetwork.h and go through the dnsproxyd socket, like it already
does for getaddrinfo().

Test: atest resolv_integration_test resolv_unit_test
Change-Id: I24d974df1096d1bd8a0c047341a260dbc5575aa4