title: Digits: user presses return without entering in phone number
body: <p>I'm trying to use Digits in my app and tested this out on a sample app:</p>
  
  <p><strong>SCREEN 1 - MAINACTIVITY:</strong></p>
  
  <p><a href="https://i.stack.imgur.com/Xks0H.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/Xks0H.png" alt="enter image description here"></a></p>
  
  <p>I press the button to use my number and it goes to the following screen:</p>
  
  <p><strong>SCREEN 2 - DIGITS SCREEN</strong></p>
  
  <p><a href="https://i.stack.imgur.com/6Ngh5.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/6Ngh5.png" alt="enter image description here"></a></p>
  
  <p>Now in my test scenario, the user does not enter in a cellphone number at all. He decides not to use my app and presses the back button at the bottom of the screen to exit from the digits screen.</p>
  
  <p>Instead of going back to <strong>SCREEN 1</strong> which is my <strong>MainActivity</strong>, I would like the app to quit.</p>
  
  <p>I'm looking for a callback method from digits to help me do so but can't seem to find one. How do I achieve the effect that I want?</p>