Accumulate network statistics based on deltas.

Network stats are now read out of the kernel in one sweep, instead of
reading per-UID.  We now accumulate the delta traffic between each
stats snapshot using the well-tested SamplingCounter pattern.

Since Wi-Fi and mobile traffic have different costs, track each
separately.  Avoids counting misc interfaces like loopback and
ethernet under total.

Bug: 5543387
Change-Id: I642004dc530113c27ef79f2abbae51d8af30117f