Add boolean flag to decide whether to use fbjni or jni

Summary:
Adds a flag useVanillaJNI in YogaConfig to determine whether to use FbJNI or JNI.
Currently default is set to false.

We will experiment based on this flag once all code have been migrated.

Reviewed By: Andrey-Mishanin

Differential Revision: D17601703

fbshipit-source-id: 377a5bd2a6f8a7584e84932e87fa7044d8165efd

java/com/facebook/yoga/YogaConfig.java

@@ -37,4 +37,8 @@ public abstract class YogaConfig {
   public abstract YogaLogger getLogger();
 
   abstract long getNativePointer();
+
+  public abstract void setUseVanillaJNI(boolean useVanillaJNI);
+
+  public abstract boolean useVanillaJNI();
 }