code: public Drawable getDrawable(int index) {
          if (index >= mLayerState.mNum) {
              throw new IndexOutOfBoundsException();
          }
          return mLayerState.mChildren[index].mDrawable;
      }
docs: Returns the drawable for the layer at the specified index.
  
  @param index The index of the layer, must be in the range {@code 0...getNumberOfLayers()-1}.
  @return The {@link Drawable} at the specified layer index.
  @attr ref android.R.styleable#LayerDrawableItem_drawable
  @see #setDrawable(int, Drawable)