@ GP Thief - The RS3 Mobile app is currently still in limited Beta testing. Jagex is currently allowing a limited number of users access to the Beta App through the Google Play store, and that limit has been reached. Downloads from any other sites, besides the Google Play store, are not "safe and verified" nor supported by Jagex.
"We will be aiming to uncap the Android Members beta in the near future so, if you’re a member and have an Android device, you’ll soon be able to play on mobile and not have to worry about getting in before the influx of other interested players."