Add Unsafe based storage backend

Summary:
@public

Adds another version of property storage for `YogaNode`, using `sun.misc.Unsafe`.

Adopts the stub concept from Litho for `Unsafe`, as it is hidden by the Android SDK.

Reviewed By: pasqualeanatriello

Differential Revision: D9140103

fbshipit-source-id: a4b376eca341b724a00f873467ae8bf8eaac69f4

java/tests/com/facebook/yoga/TestParametrization.java

@@ -59,6 +59,22 @@ public class TestParametrization {
         public String toString() {
           return "Hybrid";
         }
+      },
+      new NodeFactory() {
+        @Override
+        public YogaNode create() {
+          return new YogaNode(YogaNode.UNSAFE);
+        }
+
+        @Override
+        public YogaNode create(YogaConfig config) {
+          return new YogaNode(YogaNode.UNSAFE, config);
+        }
+
+        @Override
+        public String toString() {
+          return "Unsafe";
+        }
       });
   }