Read "qtaguid" network stats, refactor templates.

Teach NMS to read qtaguid stats from kernel, but fall back to older
stats when kernel doesn't support.  Add "tags" to NetworkStats entries
to support qtaguid.  To work around double-reporting bug, subtract
tagged stats from TAG_NONE entry.

Flesh out stronger NetworkTemplate.  All NetworkStatsService requests
now require a template, and moved matching logic into template.

Record UID stats keyed on complete NetworkIdentitySet definition,
similar to how interface stats are stored.  Since previous UID stats
didn't have iface breakdown, discard during file format upgrade.

Change-Id: I0447b5e7d205d73d28e71c889c568e536e91b8e4