Fix to MoPub MRAID support when after expanding an MRAID banner

Fix to MoPub MRAID support when after expanding an MRAID banner, the ad shows a modal view (e.g. open URL in a web view). Previously, the modal view would be shown behind the expanded ad, and the ad would not properly return to its superview when closed. Now, expansionContentView will close and be removed from the keyWindow before showing the modal view and, once the modal view is closed, expansionContentView will be re-added to the keyWindow and expanded.