title: Android: spinner with option group (no ExpandableListView)
body: <p>how is possible to create a spinner with subcategories? </p>
  
  <p>Without using ExpandableListView, just a single spinner-option group.</p>
  
  <p>Eg.</p>
  
  <pre><code>NUMBERS (unselectable)
      1
      2
      3
  LETTERS (unselectable)
      a
      b
      c
  </code></pre>