Don't create an instance of FontStylingSpan until any of the properties changed

Summary: Minor optimization/cleanup: instead of creating an instance of FontStylingSpan for every RCTVirtualText, start with a global immutable instance of use copy-on-write when any of the properties change.

Reviewed By: ahmedre

Differential Revision: D2817156

ReactAndroid/src/main/java/com/facebook/react/flat/RCTText.java

@@ -50,6 +50,7 @@ import com.facebook.react.uimanager.ViewProps;
 
   public RCTText() {
     setMeasureFunction(this);
+    getSpan().setFontSize(getDefaultFontSize());
   }
 
   @Override