Unable to programmatically tint chromecast media router button drawable

Question

I have a Toolbar on the Activity which includes cast icon.

class MyActivity : AppCompatActivity() {
   override fun onCreate(savedInstanceState: Bundle?) {
      super.onCreate(savedInstanceState)

      CastManager.getInstance(this)
   }

   override fun onCreateOptionsMenu(menu: Menu): Boolean {
      super.onCreateOptionsMenu(menu)
      menuInflater.inflate(R.menu.activity_menu, menu)
      CastManager.setupCastButtonIfSupported(this, menu)
   }
}

And the menu xml is

<menu xmlns:android="http://schemas.android.com/apk/res/android"
      xmlns:app="http://schemas.android.com/apk/res-auto">

    <item
       android:id="@+id/media_route_menu_item"
       android:title="Cast"
       app:actionProviderClass="CustomMediaRouteActionProvider"
       app:showAsAction="always"/>

</menu>

And the cast icon looks like this.

The cast icon here is picking up mediaRouteButtonTint colour defined in themes.xml. The challenge is to set tint colour programmatically. I tried using custom ActionProvider class to apply tinting.

class CustomMediaRouteActionProvider(context: Context) : MediaRouteActionProvider(context) {

   override fun onCreateMediaRouteButton(): MediaRouteButton {
      val button = super.onCreateMediaRouteButton()
      addTinting(button)

      return button
   }

   override fun getMediaRouteButton(): MediaRouteButton? {
      val button = super.getMediaRouteButton()
      addTinting(button)

      return button
   }

   private fun addTinting(button: MediaRouteButton?) {
      if(null == button) return

      val castContext: Context =
        ContextThemeWrapper(context, androidx.mediarouter.R.style.Theme_MediaRouter)

      val a = castContext.obtainStyledAttributes(
            null,
            androidx.mediarouter.R.styleable.MediaRouteButton,
            androidx.mediarouter.R.attr.mediaRouteButtonStyle,
            0
      )

      val drawable = a.getDrawable(androidx.mediarouter.R.styleable.MediaRouteButton_externalRouteEnabledDrawable)
      a.recycle()
      if (drawable != null) {
         DrawableCompat.setTintList(drawable, customDynamicColor) 
      }
      if (drawable != null) {
        drawable.state = button.drawableState
      }
      button.setRemoteIndicatorDrawable(drawable)
   }
}

However, it is not changing tinting. I'm not sure what is going wrong here. Any clue on what's breaking?