Add separate classes to implement JNI methods suing vanilla JNI

Summary:
This diffs adds a separate file YGJNIVanilla.cpp to add jni methods which uses vanilla JNI instead of FBJNI.
In this diff only one method has been added to setup the experiment boolean setup.

At the end of this diff stack , we will be able to experiment between fbjni and vanilla jni in yoga and finally get rid of fbjni which saves us around 300Kb per architecture in yoga binary size.

Reviewed By: Andrey-Mishanin

Differential Revision: D17601591

fbshipit-source-id: a88520c625bd8b5d9ffcf8ab5f02fc71dc800081

java/com/facebook/yoga/YogaNative.java

@@ -111,4 +111,8 @@ public class YogaNative {
   static native void jni_YGNodePrint(long nativePointer);
   static native void jni_YGNodeSetStyleInputs(long nativePointer, float[] styleInputsArray, int size);
   static native long jni_YGNodeClone(long nativePointer);
+
+
+  // JNI methods that use Vanilla JNI
+  public static native void jni_YGNodeStyleSetFlexJNI(long nativePointer, float flex);
 }