Move DnsProxyListener to libnetd_resolv

[1] Support APIs for netd to set callbacks and bring up
    DnsProxyListener.

[2] Keep DnsProxyListener functioning as usual by function pointers,
    including getNetworkContext(), checkCallingPermission(), and
    getPrefix64().

[3] Use libbinder_ndk to report onDnsEvent().

Test: as follows
    - built, flashed, booted
    - system/netd/tests/runtests.sh passed
    - netd_benchmark passed
    - Browsing websites passed
Change-Id: Ib6575833c248579aa079e302795b6d6cddde1f2b
diff --git a/server/ResolvStub.cpp b/server/ResolvStub.cpp
index dafd58f..774b651 100644
--- a/server/ResolvStub.cpp
+++ b/server/ResolvStub.cpp
@@ -84,6 +84,7 @@
     RESOLV_STUB_LOAD_SYMBOL(resolv_delete_private_dns_for_net);
     RESOLV_STUB_LOAD_SYMBOL(resolv_get_private_dns_status_for_net);
     RESOLV_STUB_LOAD_SYMBOL(resolv_has_nameservers);
+    RESOLV_STUB_LOAD_SYMBOL(resolv_init);
     RESOLV_STUB_LOAD_SYMBOL(resolv_register_private_dns_callback);
     RESOLV_STUB_LOAD_SYMBOL(resolv_res_nsend);
     RESOLV_STUB_LOAD_SYMBOL(resolv_set_nameservers_for_net);