Set rcode correctly for caching hit

rcode isn't set if RESOLV_CACHE_FOUND in res_nsend.
Use the rcode in cached answer to corrcet it.

Test: built, flashed, booted
      system/netd/tests/runtests.sh pas

Change-Id: Ie40331bb7c9be9a78b209fdeecc2f37965b2b30b