Allow configuring the app source folder for android

Summary:
I have been working on integrating React Native into an existing app and realised that the source folder for android code is currently hardcoded as `app`. Being a legacy codebase we can't really change that. So I wanted to be able to customise this folder name so that I can integrate RN seamlessly and start it from the CLI without having to run it from Android Studio.

- Create a barebones RN app
- `react-native run-android` works successfully
- Refactor the name of the app folder inside the android directory to `TestFolderName` using Android Studio
- `react-native run-android` now fails
- Changing this command to `react-native run-android --appFolder=TestFolderName` works successfully
Closes https://github.com/facebook/react-native/pull/13273

Differential Revision: D4833396

Pulled By: ericvicenti

fbshipit-source-id: 4c62cee6aaa2cc78eede5c7756459430022fffde

local-cli/runAndroid/runAndroid.js

@@ -86,7 +86,7 @@ function buildAndRun(args) {
     : './gradlew';
 
   const packageName = fs.readFileSync(
-      'app/src/main/AndroidManifest.xml',
+      `${args.appFolder}/src/main/AndroidManifest.xml`,
       'utf8'
     ).match(/package="(.+?)"/)[1];
 
@@ -135,7 +135,7 @@ function buildApk(gradlew) {
 
 function tryInstallAppOnDevice(args, device) {
   try {
-    const pathToApk = 'app/build/outputs/apk/app-debug.apk';
+    const pathToApk = `${args.appFolder}/build/outputs/apk/${args.appFolder}-debug.apk`;
     const adbPath = getAdbPath();
     const adbArgs = ['-s', device, 'install', pathToApk];
     console.log(chalk.bold(
@@ -288,6 +288,10 @@ module.exports = {
     description: '--flavor has been deprecated. Use --variant instead',
   }, {
     command: '--variant [string]',
+  }, {
+    command: '--appFolder [string]',
+    description: 'Specify a different application folder name for the android source.',
+    default: 'app',
   }, {
     command: '--appIdSuffix [string]',
     description: 'Specify an applicationIdSuffix to launch after build.',