BatteryStats: Compensate for accuracy issues when reporting WiFi energy data issues with Slog.wtf

In practice I've seen the reported time spent in all WiFi energy states ~20 milliseconds larger
than the time between samples, which seems reasonable.

This will reduce false positives in APR.

Bug:22774068
Change-Id: Ibf33c9826586c8a1b70449b9a609cb51bdfc08b8