Separate resolv_getaddrinfo from android_getaddrinfofornetcontext

1. Separate resolv_getaddrinfo from android_getaddrinfofornetcontext
   which only do explore_fqdn()
2. cleanup for android_getaddrinfofornetcontext/explore_fqdn

Bug: 135506574
Test: cd system/netd && atest
Change-Id: Ie5c9721f69a9c3dcf5941bd4321d076bb99c313a