@Nullable
    public static String getPersistableFolder(@NonNull final PersistableFolder folder) {
        return getString(folder.getPrefKeyId(), getLegacyValue(folder.getPrefKeyId()));
    }
gets the user-defined uri for a persistable folder. Can be null
should be called by PersistableFolder class only
main/src/main/java/cgeo/geocaching/settings/Settings.java