@Nullable
  public Bitmap updateBitmap(String id, @Nullable Bitmap bitmap) {
    ImageAssetManager bm = getImageAssetManager();
    if (bm == null) {
      Logger.warning("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;
  }
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.
lottie/src/main/java/com/airbnb/lottie/LottieDrawable.java