docs: @Nullable public Bitmap updateBitmap(String id, @Nullable Bitmap bitmap) {
      if (bitmap == null) {
        LottieImageAsset asset = imageAssets.get(id);
        Bitmap ret = asset.getBitmap();
        asset.setBitmap(null);
        return ret;
      }
      Bitmap prevBitmap = imageAssets.get(id).getBitmap();
      putBitmap(id, bitmap);
      return prevBitmap;
    }