title: Custom seekbar thumb not transparent on Lollipop API21
body: <p>This is my <strong>seekbar</strong>:</p>
  
  <pre><code>&lt;SeekBar
      android:id="@+id/seek1"
      android:layout_width="fill_parent"
      android:layout_height="wrap_content"
      android:layout_margin="10dp"
      android:progressDrawable="@drawable/style_progressbar"
      android:thumb="@drawable/style_progressbar_circle"
      android:progress="20" /&gt;
  </code></pre>
  
  <p>This is <strong>style_progressbar.xml</strong>:</p>
  
  <pre><code>&lt;layer-list xmlns:android="http://schemas.android.com/apk/res/android"&gt;
      &lt;item android:id="@android:id/background"&gt;
          &lt;shape android:shape="rectangle" &gt;
              &lt;corners android:radius="5dp" /&gt;
              &lt;gradient
                  android:angle="270"
                  android:endColor="@color/gris_hint"
                  android:startColor="@color/gris_hint" /&gt;
          &lt;/shape&gt;
      &lt;/item&gt;
      &lt;item android:id="@android:id/secondaryProgress"&gt;
          &lt;clip&gt;
              &lt;shape android:shape="rectangle" &gt;
                  &lt;corners android:radius="5dp" /&gt;
                  &lt;gradient
                      android:angle="270"
                      android:endColor="@color/gris"
                      android:startColor="@color/gris" /&gt;
              &lt;/shape&gt;
          &lt;/clip&gt;
      &lt;/item&gt;
      &lt;item android:id="@android:id/progress"&gt;
          &lt;clip&gt;
              &lt;shape android:shape="rectangle" &gt;
                  &lt;corners android:radius="5dp" /&gt;
                  &lt;gradient
                      android:angle="270"
                      android:endColor="@color/gris"
                      android:startColor="@color/gris" /&gt;
              &lt;/shape&gt;
          &lt;/clip&gt;
      &lt;/item&gt;
  &lt;/layer-list&gt;
  </code></pre>
  
  <p>And this is style_progressbar_circle.xml</p>
  
  <pre><code>&lt;selector xmlns:android="http://schemas.android.com/apk/res/android"&gt;
      &lt;item android:drawable="@drawable/red_scrubber_control_disabled_holo" android:state_enabled="false"/&gt;
      &lt;item android:drawable="@drawable/red_scrubber_control_pressed_holo" android:state_pressed="true"/&gt;
      &lt;item android:drawable="@drawable/red_scrubber_control_focused_holo" android:state_selected="true"/&gt;
      &lt;item android:drawable="@drawable/red_scrubber_control_normal_holo"/&gt;
  &lt;/selector&gt;    
  </code></pre>
  
  <p>This is how I see it in Lollipop</p>
  
  <p><img src="https://i.stack.imgur.com/JaEcF.png" alt="Seekbar Lollipop"></p>
  
  <p>This is how it should look, this is how it looks on Kitkat and lower versions.</p>
  
  <p><img src="https://i.stack.imgur.com/aXSnl.png" alt="Seekbar KitKat"></p>
  
  <p>Any idea? I've got some issues with layouts on Lollipop but this is the only one I can't solve for my own.</p>