title: How to login using username instead email on Firebase in Android application
body: <p>I am creating an application using Firebase database in which login and sign up functionality added. I want to add the login with username instead of email id.
  But I don't find any method for authentication with username. If I change firebase database rules for public then I able to login with username, but the security of database is hamper in this case. So please guide me for login with username on firebase and also suggest me on how to check username availability before sign up, if the username is available then user can sign up or if not, then the user must change his/her username for sign up. </p>