code: @Nullable
    public Bitmap updateBitmap(String id, @Nullable Bitmap bitmap) {
      ImageAssetManager bm = getImageAssetManager();
      if (bm == null) {
        Log.w(L.TAG, "Cannot update bitmap. Most likely the drawable is not added to a View " +
            "which prevents Lottie from getting a Context.");
        return null;
      }
      Bitmap ret = bm.updateBitmap(id, bitmap);
      invalidateSelf();
      return ret;
    }
docs: Allows you to modify or clear a bitmap that was loaded for an image either automatically
  through {@link #setImagesAssetsFolder(String)} or with an {@link ImageAssetDelegate}.
  
  @return the previous Bitmap or null.