bionic: Replace iface with opaque netid in resolver.

Replace iface cache key with netid.

Replace _mark with netid.

Mark sockets used to test IPv4/IPv6 support as well as sockets
used to determine source address for rfc6724 sort.

Remove pid/uid mappings (functionality moved to system/netd).

Do not create resolv_cache when it does not exist, but rather only
when DNS configuration is supplied.

Clean up unused functions.

Change-Id: I9ccfda2902cc0943e87c9bc346ad9a2578accdab