netd: Replace iface with opaque netid in resolver.

Also ensure that UID mapping (for VPN) cannot be overridden by
android_getaddrinfofornet or per-PID preference.

Change-Id: I9ccfda2902cc0943e87c9bc346ad9a2578accdab
Merge "netd: Replace iface with opaque netid in resolver."
auto-merger: merge in from aosp/master