public Uri getMapUri() {
            return mapUri;
        }
Offline maps use the hash of the filename as ID. That way changed files can easily be detected. Also we do no
longer need to differentiate between internal map sources and offline map sources, as they all just have an
numerical ID (based on the hash code).
main/src/main/java/cgeo/geocaching/maps/mapsforge/MapsforgeMapProvider.java