allow callers to specify their own Handler

It's probably pretty unusual, but sometimes we need to override the Image Loader's behavior of calling the no-argument `new Handler()` constructor to create a `Handler` for calling the `LoadAndDisplayImageTask`. The patch here allows callers to set their own `Handler` in the `DisplayImageOptions` if needed.

This is probably primarily useful if you're using `loadImage()` rather than `displayImage()`.