title: back button functionality on android with kivy
body: <p>in Kivy, when I press the back button on my android device it throws me out of the application. is there a way to return back to the previous screen using the Kivy language and not python? this is what I have written in kivy:</p>
  
  <pre><code>&lt;MyAppClass&gt;:
  AnchorLayout:
      anchor_x : 'center'
      anchor_y : 'top'
      ScreenManager:
          size_hint : 1, .9
          id: _screen_manager
  
          Screen:
              name:'screen1'
              StackLayout:
                  # irrelevant code
  
          Screen:
              name:'screen2'
              StackLayout:
                  # irrelevant code
  </code></pre>
  
  <p>I need to manipulate the screen manager and its screens from python... if I can do so I will be ok with python.</p>