Use /proc/net/dev to get stats summary

If the qtaguid proc file is no longer exist, the device is running new
eBPF module to do traffic accounting. So the NetworkStatsFactory need to
use the proc/net/dev interface to get the per interface traffic stats
summary. Also, adding a test to verify the helper function work properly

Bug: 30950746
Test: run NetworkStatsFactoryTest
Change-Id: Ia36808bf02f1637dd41a3e7c50917b91b1a77524