changes link file on Android to MainApplication.java for 0.29 update

Summary:
rnpm aka `react-native link` is broken with Android 0.29 - #8603

This gets it back to working again by checking for new MyApplication.java file, and curtailing the path when needed.
Closes facebook/react-native#8612

Differential Revision: D3533960

fbshipit-source-id: 95d799eaebb26ba1d876c88107ccd2af72427f55

Author: GantMan

rnpm/core/src/config/android/index.js

@@ -33,9 +33,9 @@ exports.projectConfig = function projectConfigAndroid(folder, userConfig) {
   const packageFolder = userConfig.packageFolder ||
     packageName.replace(/\./g, path.sep);
 
-  const mainActivityPath = path.join(
+  const mainFilePath = path.join(
     sourceDir,
-    userConfig.mainActivityPath || `src/main/java/${packageFolder}/MainActivity.java`
+    userConfig.mainFilePath || `src/main/java/${packageFolder}/MainApplication.java`
   );
 
   const stringsPath = path.join(
@@ -68,7 +68,7 @@ exports.projectConfig = function projectConfigAndroid(folder, userConfig) {
     buildGradlePath,
     settingsGradlePath,
     assetsPath,
-    mainActivityPath,
+    mainFilePath,
   };
 };