Fix protected overrides and restore IKeyFrame interface.

src/Avalonia.Animation/DoubleKeyFrames.cs

@@ -16,7 +16,7 @@ namespace Avalonia.Animation
     {
 
         /// <inheritdocs/>
-        public override double DoInterpolation(double t)
+        protected override double DoInterpolation(double t)
         {
             var pair = GetKFPairAndIntraKFTime(t);
             double y0 = pair.KFPair.FirstKeyFrame.Value;

src/Avalonia.Animation/IKeyFrames.cs

@@ -7,7 +7,7 @@ namespace Avalonia.Animation
     /// <summary>
     /// Interface for Keyframe group object
     /// </summary>
-    internal interface IKeyFrames
+    public interface IKeyFrames
     {
         /// <summary>
         /// Applies the current KeyFrame group to the specified control.