Bug 1069214 - added accessibility improvements for day/week view.

lets move this logic into the `children`, like:

```js
_setVisibleForScreenReader: function() {
  this.children.forEach(child => child.setVisibleForScreenReader(this._visibleRange));
}
```

and please note that `visibleRange.contains` can accept a `date` as argument.

```js
setVisibleForScreenReader: function(visibleRange) {
  var visible = visibleRange.contains(this.date);
  this.day.setAttribute('aria-hidden', !visible);
  this.allday.setAttribute('aria-hidden', !visible);
}
```