Use task completion source for room join flow

On Android, users were unable to join or create multiplayer rooms. The
root cause of that was that the both the wait and set of the
`ManualResetEvent` in `getRoomUsers` occurred on the same thread, which
created a chicken-and-egg situation - the set could not proceed until
the wait had actually completed.

Resolve by substituting the `ManualResetEvent` for a
`TaskCompletionSource` to achieve a promise-style task, which the
previous code was a crude approximation of anyway.

Closes #11385.

osu.Game/Online/Multiplayer/StatefulMultiplayerClient.cs

@@ -7,7 +7,6 @@
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.Linq;
-using System.Threading;
 using System.Threading.Tasks;
 using osu.Framework.Allocation;
 using osu.Framework.Bindables;
@@ -128,7 +127,8 @@ public async Task JoinRoom(Room room)
 
             Debug.Assert(Room != null);
 
-            var users = getRoomUsers();
+            var users = await getRoomUsers();
+            Debug.Assert(users != null);
 
             await Task.WhenAll(users.Select(PopulateUser));
 
@@ -437,24 +437,20 @@ Task IMultiplayerClient.ResultsReady()
         /// This should be used whenever accessing users from outside of an Update thread context (ie. when not calling <see cref="Drawable.Schedule"/>).
         /// </summary>
         /// <returns>A copy of users in the current room, or null if unavailable.</returns>
-        private List<MultiplayerRoomUser>? getRoomUsers()
+        private Task<List<MultiplayerRoomUser>?> getRoomUsers()
         {
-            List<MultiplayerRoomUser>? users = null;
-
-            ManualResetEventSlim resetEvent = new ManualResetEventSlim();
+            var tcs = new TaskCompletionSource<List<MultiplayerRoomUser>?>();
 
             // at some point we probably want to replace all these schedule calls with Room.LockForUpdate.
             // for now, as this would require quite some consideration due to the number of accesses to the room instance,
             // let's just add a manual schedule for the non-scheduled usages instead.
             Scheduler.Add(() =>
             {
-                users = Room?.Users.ToList();
-                resetEvent.Set();
+                var users = Room?.Users.ToList();
+                tcs.SetResult(users);
             }, false);
 
-            resetEvent.Wait(100);
-
-            return users;
+            return tcs.Task;
         }
 
         /// <summary>