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Like others pointed FileStorage is not available on Android, so in order to avoid using the Android NDK I just save it to a Bitmap.

Here's the code I use:

    /**
 * Saves a Mat to the SD card application folder as a jpg.  
 * 
 * @param source The image to save.
 * @param filename The name of the file to be saved.
 * @param directoryName The directory where the 
 * @param ctx The activity context.
 * @param colorConversion The openCV color conversion to apply to the image. -1 will use no color conversion.
 */
public void saveImageToDisk(Mat source, String filename, String directoryName, Context ctx, int colorConversion){

    Mat mat = source.clone();
    if(colorConversion != -1)
        Imgproc.cvtColor(mat, mat, colorConversion, 4);

    Bitmap bmpOut = Bitmap.createBitmap(mat.cols(), mat.rows(), Bitmap.Config.ARGB_8888);
    Utils.matToBitmap(mat, bmpOut);
    if (bmpOut != null){

        mat.release();
        OutputStream fout = null;
        String root = Environment.getExternalStorageDirectory().getAbsolutePath();
        String dir = root + "/" + ctx.getResources().getString(R.string.app_name) + "/" + directoryName;
        String fileName = filename + ".jpg";
        File file = new File(dir);
        file.mkdirs();
        file = new File(dir, fileName);

        try {
            fout = new FileOutputStream(file);
            BufferedOutputStream bos = new BufferedOutputStream(fout);
            bmpOut.compress(Bitmap.CompressFormat.JPEG, 100, bos);
            bos.flush();
            bos.close();
            bmpOut.recycle();
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        }

        catch (IOException e) {
            e.printStackTrace();
        }
    }
    bmpOut.recycle();
}

Hope it helps someone.