Improve how battery stats collects network stats.

This optimizes the path for battery stats to collect
per-uid network usage.  It now collects wifi and mobile
usage separately, with a path that allows it to recycle
all data structures and filter out stats it isn't
interested in before they come back to java.

This is setting us up for the actual goal, to collect
mobile stats independently each time the mobile radio
goes down, allowing us to distribute mobile radio usage
across uids based on the number of packets they transferred
during a session.

Change-Id: I21a0f517cf087ea5aa8b8dd535e20b46e361a52b