Expose ReactCommon/cxxreact headers via Prefab

Summary:
Reference reactwg/react-native-releases#41 (reply in thread)
I'm exposing the headers inside `ReactCommon/cxxreact` to be access via the Prefab API.

Changelog:
[Internal] [Changed] - Expose ReactCommon/cxxreact headers via Prefab

Reviewed By: cipolleschi

Differential Revision: D41965511

fbshipit-source-id: 726b2948262b4b07cb40996fa1ccf9726a5197ce

ReactAndroid/build.gradle

@@ -147,6 +147,7 @@ final def preparePrefab = tasks.register("preparePrefab", PreparePrefabHeadersTa
                     new Pair(new File(buildDir, "third-party-ndk/glog/exported/").absolutePath, ""),
                     new Pair("../ReactCommon/butter/", "butter/"),
                     new Pair("../ReactCommon/callinvoker/", ""),
+                    new Pair("../ReactCommon/cxxreact/", "cxxreact/"),
                     new Pair("../ReactCommon/react/bridging/", "react/bridging/"),
                     new Pair("../ReactCommon/react/config/", "react/config/"),
                     new Pair("../ReactCommon/react/nativemodule/core/", ""),