Call spectator EndPlaying() immediately after score submission

As it turns out, in current `master`, if a gameplay session ends
normally (i.e. by the player completing the beatmap in full), then
the spectator server `EndPlaying()` method will not be called until
`SubmittingPlayer.OnExiting()`, which in practice turns out to be
the moment where the user exits from the post-gameplay results screen
back to song select.

There is seemingly no reasonable cause for not calling this earlier. In
fact the solo spectator flow looks more broken without this call than
with, because without it the spectator view just hangs until the
spectated player exits gameplay, and *only then* shows results, rather
than do it upon normal gameplay completion.

osu.Game/Screens/Play/SubmittingPlayer.cs

@@ -130,6 +130,7 @@ namespace osu.Game.Screens.Play
             score.ScoreInfo.Date = DateTimeOffset.Now;
 
             await submitScore(score).ConfigureAwait(false);
+            spectatorClient.EndPlaying(GameplayState);
         }
 
         [Resolved]