Improve safety of ongoing operation tracker

Finishing an operation started via
`OngoingOperationTracker.BeginOperation()` was risky in cases where the
operation ended at a callback on another thread (which, in the case of
multiplayer, is *most* cases). In particular, if any consumer registered
a callback that mutates transforms when the operation ends, it would
result in crashes after the framework-side safety checks.

Rework `OngoingOperationTracker` into an always-present component
residing in the drawable hierarchy, and ensure that the
`operationInProgress` bindable is always updated on the update thread.
This way consumers don't have to add local schedules in multiple places.

osu.Game/Screens/OnlinePlay/OnlinePlayScreen.cs

@@ -54,7 +54,7 @@ namespace osu.Game.Screens.OnlinePlay
         private readonly Bindable<FilterCriteria> currentFilter = new Bindable<FilterCriteria>(new FilterCriteria());
 
         [Cached]
-        private readonly OngoingOperationTracker ongoingOperationTracker = new OngoingOperationTracker();
+        private OngoingOperationTracker ongoingOperationTracker { get; set; }
 
         [Resolved(CanBeNull = true)]
         private MusicController music { get; set; }
@@ -144,7 +144,8 @@ namespace osu.Game.Screens.OnlinePlay
                         };
                         button.Action = () => OpenNewRoom();
                     }),
-                    RoomManager = CreateRoomManager()
+                    RoomManager = CreateRoomManager(),
+                    ongoingOperationTracker = new OngoingOperationTracker()
                 }
             };