Move the playstate handling to the State machine instead of being pipelined by the timer itself.

Removed unused AnimationStateTimer.

src/Avalonia.Animation/Animatable.cs

@@ -74,4 +74,4 @@ namespace Avalonia.Animation
             }
         }
     }
-}
+}

src/Avalonia.Animation/Timing.cs

@@ -12,7 +12,7 @@ namespace Avalonia.Animation
     /// <summary>
     /// Provides global timing functions for animations.
     /// </summary>
-    public static class Timing
+    public class Timing
     {
         static TimerObservable _timer = new TimerObservable();