Full screen ads are nothing new, but they would be part of what ever app you were using. That includes exiting, if you are using the exit function of a free2play game it can show you a full screen ad that you would need to close before the app will actually send you back to the home screen.
You should NOT however get full screen ads when flicking through your apps, or using any of the system integrated apps like setup, or googles own email/calendar/SMS apps. (None of googles apps will do full screen ads, accept chrome/browser in which full screen ads would be part of the web site you are viewing.)