public GenericDraweeHierarchyBuilder setOverlay(@Nullable Drawable overlay) {
    if (overlay == null) {
      mOverlays = null;
    } else {
      mOverlays = Arrays.asList(overlay);
    }
    return this;
  }
Sets a single overlay.

@param overlay overlay drawable
@return modified instance of this builder
drawee/src/main/java/com/facebook/drawee/generic/GenericDraweeHierarchyBuilder.java