Move ResponseCode to libnetdutils

ResponseCode is necessary for libnetd_resolv, move it to libnetdutils
to ease the cleanup of the include path system/netd/server for
libnetd_resolv.

Bug: 128662167
Test: system/netd/tests/runtests.sh passed

Change-Id: Iae22cc6b4c642a190294fa4ce0ae406434e7ac3d