Extract abstract class YogaNode

Summary:
@public

Here, we extract an abstract class from `YogaNode`, in order to allow for experimentation with different implementations.

The reason for not choosing an interface is to keep ABI compatibility for `YogaNode.create()`.

Reviewed By: pasqualeanatriello

Differential Revision: D14044990

fbshipit-source-id: f8eb246338b55f34f0401198c0655abfcb7c9f37

java/com/facebook/yoga/YogaConfig.java

@@ -106,7 +106,7 @@ public class YogaConfig {
   }
 
   @DoNotStrip
-  private final YogaNode cloneNode(YogaNode oldNode, YogaNode parent, int childIndex) {
-    return mYogaNodeCloneFunction.cloneNode(oldNode, parent, childIndex);
+  private final YogaNodeJNI cloneNode(YogaNodeJNI oldNode, YogaNodeJNI parent, int childIndex) {
+    return (YogaNodeJNI) mYogaNodeCloneFunction.cloneNode(oldNode, parent, childIndex);
   }
 }