public void start(CameraSource cameraSource, GraphicOverlay graphicOverlay)
      throws SecurityException {
    this.graphicOverlay = graphicOverlay;
    if (cameraSource != null) {
      this.cameraSource = cameraSource;
      startRequested = true;
      startIfReady();
    } else {
      release();
    }
  }
Starts the display of the camera preview. If the camera source passed to this method is null,
then the previous camera source is stopped and its resources get released.

@param cameraSource the camera source from where the preview frames come from.
@param graphicOverlay CameraSourcePreview will set the preview info to the GraphicOverlay
instance when available.
@throws SecurityException if there is no permission to use the camera when this method was
called.
java/com/google/android/apps/authenticator/barcode/preview/CameraSourcePreview.java