Make TrainingAnalyticsSideChannel internal (#4999)

com.unity.ml-agents/Runtime/SideChannels/TrainingAnalyticsSideChannel.cs

     /// <summary>
     /// Side Channel implementation for recording which training features are being used.
     /// </summary>
-    public class TrainingAnalyticsSideChannel : SideChannel
+    internal class TrainingAnalyticsSideChannel : SideChannel
     {
         const string k_TrainingAnalyticsConfigId = "b664a4a9-d86f-5a5f-95cb-e8353a7e8356";