You can find out which app is currently opened in Android. Do this repeatedly and you get an impression for how long which app gets used. All App Annie needs to

You can find out which app is currently opened in Android. Do this repeatedly and you get an impression for how long which app gets used. All App Annie needs to do is pay popular app vendors to ship their data collection SDK. They might not get perfect data like Google does, but large enough samples to make statements like this.