[Layout] Improve Layout System Abstraction (#2941)

* Improve Layout Abstraction

* Address naming comments

- Rename ASPrimitiveTraitEnvironment to ASTraitEnvironment
- Rename ASPrimitiveTraitCollectionPropagateDown to ASTraitCollectionPropagateDown
- Rename progagateNewPrimitiveTraitCollection: to propagateNewTraitCollection:

AsyncDisplayKit/Layout/ASLayoutElementExtensibility.h

@@ -10,7 +10,11 @@
 //  of patent rights can be found in the PATENTS file in the same directory.
 //
 
+#if AS_TARGET_OS_IOS
 #import <UIKit/UIGeometry.h>
+#else
+#import <Foundation/NSGeometry.h>
+#endif
 
 @protocol ASLayoutElementExtensibility <NSObject>
 
@@ -26,4 +30,5 @@
 - (void)setLayoutOptionExtensionEdgeInsets:(UIEdgeInsets)value atIndex:(int)idx;
 - (UIEdgeInsets)layoutOptionExtensionEdgeInsetsAtIndex:(int)idx;
 
+
 @end