Unify DNS error code for android_gethostbyaddrfornetcontext

Both gethostbyname and getaddrinfo return error code POSIX EAI_* now. Unify
the error codes of gethostbyaddr to be POSIX EAI_* as well.

Test: system/netd/tests/runtests.sh passed
Change-Id: Iee6963e2ebd21fd0c446b487784eb59af37a544e