MediaRouter: Use a fixed height for individual volume sliders

Change-Id: Ia73b5eef615ed8c9f42833d41dccc41d798924c3
diff --git a/v7/mediarouter/res/layout/mr_controller_volume_item.xml b/v7/mediarouter/res/layout/mr_controller_volume_item.xml
index 3b6ce71..4d693e2 100644
--- a/v7/mediarouter/res/layout/mr_controller_volume_item.xml
+++ b/v7/mediarouter/res/layout/mr_controller_volume_item.xml
@@ -39,7 +39,7 @@
                    android:src="?attr/mediaRouteAudioTrackDrawable" />
         <android.support.v7.app.MediaRouteVolumeSlider android:id="@+id/mr_volume_slider"
                  android:layout_width="fill_parent"
-                 android:layout_height="wrap_content"
+                 android:layout_height="40dp"
                  android:minHeight="40dp"
                  android:maxHeight="40dp" />
     </LinearLayout>