https://arstechnica.com/information-technology/2019/08/85-google-play-apps-with-8-million-downloads-forced-fullscreen-ads-on-users/

Researchers found 85 Google Play apps with more than 8 million downloads that forced users to view fullscreen ads.

The apps, which posed as photography and gaming programs, contained a family of adware that was highly disruptive to end users. Once installed, the apps displayed ads in full screen—a setting that forced users to view the entire duration of an ad before being able to close the window or get back to the app. The apps showed an ad every five minutes, but the people operating the platform had the ability to remotely change the frequency.