Fix in Wifi activity updates

When Wifi firmware resets link layer stats, it sometimes returns a value
for On time which is much larger than the duration of the report.
On time = Idle time + Rx time + Tx time.
This change prevents those errors from affecting Wifi power metrics.

BUG:110325882
Test:
adb shell dumpsys batterystats
WiFi Scan time:  12s 302ms
WiFi Idle time:   5m 48s 634ms
WiFi Rx time:     24s 232ms
WiFi Tx time:     10s 510ms

Change-Id: I7b69fc728b275f89154dd0c1d7382576ebfadc29