Persist network stats using AtomicFile.

Implements read/write of network stats using AtomicFile, along with
magic number and versioning.  Stores in "/data/system/netstats.bin"
for now.  Tests to verify that stats are persisted across a simulated
reboot, and to verify that TEMPLATE_WIFI is working.

Fixed bug where kernel counters rolling backwards would cause negative
stats to be recorded; now we clamp deltas at 0.

Change-Id: I53bce26fc8fd3f4ab1e34ce135d302edfa34db34