ENF: Fix average RSSI calculation

[transistortim]

Correctly discard new measurements when the reported timestamp is older
than the timestamp in the database.
Fixes #1655.

[Elsensee]

Also, the first RSSI value is always discarded, because MINIMUM_EXPOSURE_DURATION_MS is 0 and there is rssi * timestamp (=0).
Maybe that should be set to 250? (Advertisements are sent every 250ms, so I would assume that this is the minimal value)

[mar-v-in]

Indeed the first value is discarded, but that doesn't really matter. What matters is the time between the scan periods (3-5 minutes), where we do not have any RSSI data (as we are not scanning) so we are approximating it based on the first occurrence in the following scan period. This is similar to the behavior in GMS. (IIRC they approximate based on the previous scan period, which would mean they effectively discard the last measured rssi value.)