title: Returning users can login with pin code instead of username/password
body: <p>I have a pretty basic app that uses Firebase.io for user authorization. I've found some things on how to set up so you can recognize returning users and start them on a different screen as first time users by setting a Boolean value in <code>getSharedPreferences</code>. 
  I've seen apps that do this, like Discover (a banking app) allows you to set up a 4 digit pin to login with instead of always entering your username and password. 
  Does anyone know how this is achieved, where I can look for a tutorial or more info on it? </p>
  
  <p>My first idea was that you would store the pin in <code>getSharedPreferences</code> but I couldn't find anything about that and I'm not sure how secure that would be. </p>