For the last year or so, Microsoft has offered Xbox owners a subscription option called Xbox Game Pass. It comes with unlimited access to more than 100 Xbox One and Xbox 360 games, as well as some other perks. Now, you can manage your Game Pass account and content from the comfort of your Android phone by downloading the new Game Pass app in the Play Store.
To be completely clear, this app does not have anything to do with playing games on your phone. It lists all the games available with Xbox Game Pass for your console and can push notifications when new titles are added to the service. You can also use the app to purchase games on your account with a 20% discount (another perk of the subscription). There's also a beta feature to remotely install new Game Pass titles to your Xbox.
If you don't have a Game Pass subscription, the app will still let you browse around. If you want to sign up, the app directs you to the web, probably to avoid giving Google a cut of the subscription. Game Pass runs $10 per month, but you can get a free trial and your first month for $2. The app is also available on APK Mirror.