Report sparse per-iface stats using atrace.

This reports the total bytes sent and received per interface using
atrace. The totals are taken from the interface counters, rather than
the ringbuffer, since the ring buffer can run out of space.

Interfaces are only reported for interfaces which have packets
sent on them as identified by Nettrace. This helps reduce the cost
significantly for idle periods and on devices with a large number of
interfaces accumulated in the iface stats maps.

Test: TreeHugger
Change-Id: I0e6a3b11de0d1e153b9f8ebb97c338f425381a9c