TrafficMeter: double-check traffic stats

On some old devices(like my poor MI-ONE Plus), the traffic stats used to
stick on 0B/s forever. This patch does a workaround to this bug by
checking traffic stats from /proc/net/dev when the Android interface
returns zero.

Revised by C3C0:
- fixed identation (we use spaces instead of tabs)
- buffered reader needs to be closed explicitly, otherwise leaks may
occur