Network stats with varint, omit parcel fields.

Persist NetworkStatsHistory using variable-length encoding; since
most buckets have small numbers, we can encode them tighter. Initial
test showed 44% space savings. Also persist packet and operation
counters.

Let NetworkStatsHistory consumers request which fields they actually
need to reduce parcel overhead.

Tests for verify varint and history field requests, also verify end-
to-end by persisting history into byte[] and restoring. Expose
bandwidth control enabled state. Extend random generation to create
packet and operation counts. Moved operation counts to long.

Fix bug that miscalculated bytes since last persist, which would
cause partial stats loss when battery pulled.

Bug: 4581977, 5023706, 5023635, 5096903
Change-Id: If61e89f681ffa11fe5711471fd9f7c238d3d37b0