Support read iface stats detail

The networkStatsFactory need to parse the detail traffic stats of each
interface instead of the total number. This change added a helper
function to return a vector of stats_line contain stats for each iface.

Bug: 72111305
Test: ./libbpf_test
Change-Id: I9a9cda7ab90cf533c2f2cc81b37b4d520d442ce2