finalize ManualRevealCallback class

I'm an idiot but I often start off with an AsyncCallback and then convert it to a ManualReveal callback and end up with:

```
new ManualRevealCallback<T>(aPresenter) {
    @Override
    public void onFailure(Throwable caught) {

     }

     @Override
     public void onSuccess(T result) {
          //do some work and then wonder why the presenter is never revealed.
     }
}
```

This prevents the above by preventing ManualRevealCallback from being subclassed.