Fix idle tracker assuming time starts at 0

`IdleTracker` in its construction quietly assumed that the clock it
receives from its parent starts ticking from 0 at the point at which it
is passed down. This is not necessarily the case when headless
executions are involved, which means that the initial state of the
tracker could be computed as idle incorrectly.

Resolve by explicitly reading the clock time at the point of
`LoadComplete()`.

osu.Game/Input/IdleTracker.cs

@@ -42,6 +42,12 @@ namespace osu.Game.Input
             RelativeSizeAxes = Axes.Both;
         }
 
+        protected override void LoadComplete()
+        {
+            base.LoadComplete();
+            updateLastInteractionTime();
+        }
+
         protected override void Update()
         {
             base.Update();