Improve TrafficStats UID APIs.

Deprecate transport layer statistics, leaving only the summarized
network layer statistics.

Improve documentation to be clear about layers where measurements
occur, and their behavior since boot.  Under the hood, move to using
xt_qtaguid UID statistics.

Bug: 6818637, 7013662
Change-Id: I9f26992e5fcdebd88c671e5765bd91229e7b0016