Bug 1047283 - [Calendar] Drawer /Add event/ Display event should not play scrolling animation

I would probably simplify this like:

```js
_shouldScroll: function() {
  return !(this._previousDay &&
    isSameDate(this.timeController.day, this._previousDay));
}
```

and keep the `this._previousDay` logic elsewhere (probably on the `onactive`).. That way we follow the [Command-Query Separation](http://en.wikipedia.org/wiki/Command%E2%80%93query_separation)