Instantiate highlight background container on animation

Also removes the necessity of scheduling as it actually never worked as
intended, `Scheduler` will still update even when the chat line is
masked away, and the animation will never be held anyways.

The new duration of the animation should be enough for long scrolls
either way.

osu.Game/Overlays/Chat/DrawableChannel.cs

@@ -102,7 +102,7 @@ namespace osu.Game.Overlays.Chat
                     {
                         float center = scroll.GetChildPosInContent(chatLine, chatLine.DrawSize / 2) - scroll.DisplayableContent / 2;
                         scroll.ScrollTo(Math.Clamp(center, 0, scroll.ScrollableExtent));
-                        chatLine.ScheduleHighlight();
+                        chatLine.Highlight();
                     }
 
                     highlightedMessage.Value = null;