title: After migration to AndroidX android:button is not respected for API below Lollipop
body: <p>I have very simple checkbox:</p>
  
  <pre><code>&lt;CheckBox xmlns:android="http://schemas.android.com/apk/res/android"
      android:id="@+id/clipboardBtn"
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:button="@drawable/clipboard_checkbox" /&gt; 
  </code></pre>
  
  <p>For checked/unchecked I have two different images.</p>
  
  <p>After migration to AndroidX I see default image for Android on devices below API 21.</p>
  
  <p>What I've tried already:</p>
  
  <ul>
  <li>change CheckBox to AppCompatCheckbox (from AndroidX) - nothing changed</li>
  <li>set background to checkbox &amp; set <code>android:button="@null"</code> - background is OK, but I still see default image on background (see image below)</li>
  </ul>
  
  <p>Seems that Android completly disrespect <code>button</code> attribute.</p>
  
  <p><a href="https://i.stack.imgur.com/ga7n7.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/ga7n7.png" alt="example of background set to img and button to null"></a></p>
  
  <p>I am out of ideas. For Lollipop+ everythink works as it should. Have anyone faced issue like this?
  Only change I did was migration to AndroidX :/</p>