title: using python 2.7 with kivy slider widget will not display background image on android
body: <p>I'm using Kivy 1.9.2 with buildozer .32. I have a slider widget with a custom background image and a custom cursor image. They both display correctly on my computer, but when I push the apk to my phone (Android 6.0.1), the slider widget displays using the default images.</p>
  
  <p><strong>from main.py:</strong></p>
  
  <pre><code>class MenuSlider(Slider): pass 
  </code></pre>
  
  <p><strong>from layout.kv:</strong></p>
  
  <pre><code>&lt;MenuSlider&gt;:
      size_hint_x: 0.5
      background_horizontal: 'resources/graphics/UI/sliderbackground.png'
      cursor_image: 'resources/graphics/UI/slidercursor.png'
  </code></pre>
  
  <p><strong>from buildozer.spec:</strong></p>
  
  <pre><code>android.api = 22
  </code></pre>