Begin collecting xtables iface counters.

Add method to parse new iface_stat_fmt proc stats, or return null
when kernel support is unavailable. Add test and remove older, unused
parsing code. Create new "xt" recorder to persist the new xtables
counters when available.

Add SSID support to NetworkIdentity to fix policy tests.

Bug: 6422414
Change-Id: I77f70e9acb79a559ab626f3af5c4f3599801ed43