Fix native modules linking in 0.29.1

Summary:
Attempt to fix https://github.com/facebook/react-native/pull/8612

We re-named `mainActivityPath` by `mainFilePath` in the `link` code, but we forgot to rename config parameters. Currently, link is broken.

- [x] `react-native link` should work for react-native 0.29+
Closes https://github.com/facebook/react-native/pull/8807

Differential Revision: D3576176

fbshipit-source-id: 60ecbd660563923696bbef1ed3b0900a7d58469f

local-cli/rnpm/link/src/android/registerNativeModule.js

@@ -27,12 +27,12 @@ module.exports = function registerNativeAndroidModule(
   applyPatch(projectConfig.stringsPath, makeStringsPatch(params, name));
 
   applyPatch(
-    projectConfig.mainActivityPath,
+    projectConfig.mainFilePath,
     makePackagePatch(androidConfig.packageInstance, params, name)
   );
 
   applyPatch(
-    projectConfig.mainActivityPath,
+    projectConfig.mainFilePath,
     makeImportPatch(androidConfig.packageImportPath)
   );
 };