title: Setting backgroundTint on Material Button with alpha has a weird visual effect in normal and pressed state
body: <p>I'm using Material Buttons in my project and trying to set backgroundTint with alpha value.  </p>
  
  <pre><code>&lt;!-- background_tint.xml --&gt;
  &lt;selector xmlns:android="http://schemas.android.com/apk/res/android"&gt;
      &lt;item android:color="@android:color/black" android:alpha="0.60"/&gt;
  &lt;/selector&gt;
  
  &lt;!-- activity_main.xml --&gt;
  &lt;LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
      android:orientation="vertical"
      android:layout_width="match_parent"
      android:layout_height="match_parent"&gt;
  
      &lt;com.google.android.material.button.MaterialButton
          android:layout_width="match_parent"
          android:layout_height="wrap_content"
          android:layout_margin="16dp"
          android:text="Sign in"
          app:backgroundTint="@color/background_tint" /&gt;
  &lt;/LinearLayout&gt;
  </code></pre>
  
  <p>The resulting button however looks weird while in normal state and even weirder while pressed.<br>
  <a href="https://i.stack.imgur.com/0dpa6.png" rel="noreferrer"><img src="https://i.stack.imgur.com/0dpa6.png" alt="enter image description here"></a><br>
  I don't see this issue when I set backgroundTint to specific shade of gray such as #777777. Why does this happen with alpha value?</p>