@Nullable
    public static String getPersistableUri(@NonNull final PersistableUri persistedUri) {
        return getString(persistedUri.getPrefKeyId(), getLegacyValue(persistedUri.getPrefKeyId()));
    }
gets the persisted uri for a persistable uris. Can be null
should be called by PersistableUri class only
main/src/main/java/cgeo/geocaching/settings/Settings.java