Inroduce BoxComponent

Summary:
The goal of this diff is to introduce the scafolding of a BoxComponent. This class will be eventually shared by Android and iOS

changelog: [internal] internal

Reviewed By: sammy-SC

Differential Revision: D41205801

fbshipit-source-id: e33e58062aa33e0c8e9a48fd112309a358f0a060

ReactAndroid/src/main/jni/react/cxxcomponents/ComponentManager.h

@@ -21,7 +21,7 @@ class ComponentManager {
       Tag tag,
       Props::Shared initialProps) = 0;
 
-  virtual std::shared_ptr<Component> createComponent(Tag tag) = 0;
+  virtual std::unique_ptr<Component> createComponent(Tag tag) = 0;
 
   virtual ~ComponentManager() = default;
 };