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
3f3913550c