Google has changed the way banner ads work in Android apps in an effort to cut down on mistaken clicks. From now on, clicking around the outside border of a banner ad will prompt a confirmation that requires you to acknowledge that you actually want to visit the website the ad is associated with.
This feature is very welcome for apps that place banner ads very close to real content that you actually want to click on. Third-party ad-supported Twitter clients are especially guilty of this. It can be very annoying to close an ad only to click on it when it suddenly reappears.
Google believes this change will improve the experience of users, developers and advertisers alike.
Source [Google Mobile Ads Blog]