Update an Android appcompat media reference to AndroidX.

This one slipped through the cracks when the rest were updated in
63d5fba9

Bug: 1126716
Change-Id: Ia67f0f60a4f7bcc89c98a550f16dd31974e80f56
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2402208Reviewed-by: Yaron Friedman <yfriedman@chromium.org>
Commit-Queue: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805783}

chrome/android/features/media_router/java/res/layout/caf_controller_media_route_button.xml

@@ -3,7 +3,7 @@
      Use of this source code is governed by a BSD-style license that can be
      found in the LICENSE file.
 -->
-<androidx.appcompat.app.MediaRouteButton
+<androidx.mediarouter.app.MediaRouteButton
     xmlns:android="http://schemas.android.com/apk/res/android"
     android:id="@+id/cast_controller_media_route_button"
     android:layout_width="wrap_content"