title: Log a user in using a generated Anonymous Login ID
body: <p>I am writing an Android application and I am trying to log users in anonymously so they don't have to go through any sort of registration process. I am storing their anonymous user ID in shared preferences, and when the application opens, I am trying to log them in based on that user ID. I am trying to figure out the correct way to do this, as there doesn't seem to be an auth function that just takes in a UID. Currently I have it using auth(), but I don't feel like that is correct.</p>
  
  <p>Here is some sample code:</p>
  
  <pre><code>String userID = getUserID();
  
      if(userID.equals("NOT_FOUND")) {
          ref.authAnonymously(new Firebase.AuthResultHandler() {
              @Override
              public void onAuthenticated(AuthData authData) {
                  //successful authentication
                  //save auth data
                  SharedPreferences prefs = getSharedPreferences(
                          "USER_ID", Context.MODE_PRIVATE);
                  String id = authData.getUid();
                  prefs.edit().putString("USER_ID", id).commit();
              }
  
              @Override
              public void onAuthenticationError(FirebaseError firebaseError) {
                  //unsuccessful authentication
              }
          });
      } else {
          ref.auth(userID, new Firebase.AuthListener() {
             ...
  </code></pre>