Database: Add C/N0 stats to GPSTest Database

[barbeau]

Is your feature request related to a problem? Please describe.

As described in https://issuetracker.google.com/312680760, some devices report C/N0 values that are much higher than other devices. It would be nice to know which devices have this issue.

Describe the solution you'd like

In each GPSTest Database record we should upload the max, min, and avg C/N0 values. This would give us a rough dataset to identify outlier devices that report C/N0 values much higher than others.