title: Setting alpha of ColorDrawable does not work
body: <p>I am trying to achieve an effect in my app of where the user scrolls down, the opacity of a view will change from 0 to 1.</p>
  
  <p>To achieve this, I've created a ColorDrawable with my desired color, blue, and then set its alpha to 0.</p>
  
  <pre><code>val actionBarBackground = ColorDrawable(ContextCompat.getColor(it, R.color.myBlue))
  (activity as? AppCompatActivity)?.supportActionBar?.setBackgroundDrawable(actionBarBackground)
  </code></pre>
  
  <p>However, after increasing the alpha, it does not change. I've tried printing the value of actionBarBackground but its still 0...</p>
  
  <pre><code>// This is called inside a scrollview callback that calculates an alpha value between 0 and 255
  actionBarBackground.alpha = 255
  Log.d(TAG, "Alpha: ${actionBarBackground.alpha}") // Prints: Alpha: 0
  </code></pre>
  
  <p>Any ideas why the alpha of the <code>ColorDwarable</code> does not change? Thank you.</p>