viewSelector: Use onStopped callback when fading out pages

With 8b368d01 we fixed a bug where the onComplete callback was always called no matter whether the transition was interrupted before or not. This exposed another bug: viewSelector depends on this behaviour when fading out pages: After fading out a page, we call this._animateIn to show the new page. Now if the fade-out animation gets interrupted, with the correct behaviour of onComplete we end up not showing a new page and the viewSelector remains empty instead. One case where this happens is when pressing a key to start a search during the overview-animation.

Obviously we also want to show the new page in case the fade-out animation was interrupted, so use the onStopped callback instead of the onComplete callback here.