android: settings' storage usage != system's storage usage

[bassosimone]

I just noticed that the storage usage according to the OONI settings does not match the storage usage according to the Android system settings. In my test case, OONI says it's using 5 MiB of storage, while Android says it's using 19 MiB.

We should investigate why that is the case, and whether we could improve out estimate. I am also wondering whether we should not report the storage we think we are using and let the system do this job (which probably it does better than us).

Another possible fix could be to say that the number we report on is just the database size (which is what we're reporting according to https://github.com/ooni/probe-android/blob/master/app/src/main/java/org/openobservatory/ooniprobe/model/database/Measurement.java#L205). Even just a change in the copy would be an improvement, I guess.