@Nullable
  public Bitmap updateBitmap(String id, @Nullable Bitmap bitmap) {
    return lottieDrawable.updateBitmap(id, bitmap);
  }
Allows you to modify or clear a bitmap that was loaded for an image either automatically
through {@link #setImageAssetsFolder(String)} or with an {@link ImageAssetDelegate}.

@return the previous Bitmap or null.
lottie/src/main/java/com/airbnb/lottie/LottieAnimationView.java