rxjava-android: parameterize OperatorViewClick by concrete view type

Parameterize OperatorViewClick observable to actual View type.
So actual view with it native type can be used.

So instead of 

```
Observable<View> imageClicks = ViewObservable.clicks(imageView, false);
        //...
        imageClicks.subscribe(new Action1<View>() {
            @Override
            public void call(View view) {
                ImageView imageView = (ImageView) view;
                imageView.setImageBitmap(bitmap);
            }
        });
```

you can use

```
        Observable<ImageView> imageClicks = ViewObservable.clicks(imageView, false);
        //...
        imageClicks.subscribe(new Action1<ImageView>() {
            @Override
            public void call(ImageView view) {
                view.setImageBitmap(bitmap);
            }
        });
```