amplify-js: [react-native] Android builds fail out of the box (with workarounds)

At the moment, the documentation for react-native will not work for android builds.

It simply says to add the NPM packages then react-native link:

> react-native init SomeProject
...
> cd SomeProject
> awsmobile init <existing-project-guid>
...
> yarn add aws-amplify aws-amplify-react-native  # already added by awsmobile init, but being clear
> react-native link
Scanning folders for symlinks in C:\code\skilitics\dev\react-native\MultiLogin\node_modules (22ms)
rnpm-install info Linking aws-amplify-react-native android dependency
rnpm-install info Platform 'android' module aws-amplify-react-native has been successfully linked

but this will cause react-native run-android to fail with:

A problem occurred configuring project ':app'.
> Could not resolve all dependencies for configuration ':app:_debugApkCopy'.
   > Could not find com.google.firebase:firebase-messaging:11.8.0.
     Required by:
         MultiLogin:app:unspecified > MultiLogin:aws-amplify-react-native:unspecified
   > Could not find com.google.firebase:firebase-core:11.8.0.
     Required by:
         MultiLogin:app:unspecified > MultiLogin:aws-amplify-react-native:unspecified

This seems to be because the allprojects section only applies from the top-level build.gradle, so the user must also add the google maven repository:

 allprojects {
     repositories {
         mavenLocal()
         jcenter()
         maven {
             // All of React Native (JS, Obj-C sources, Android binaries) is installed from npm
             url "$rootDir/../node_modules/react-native/android"
         }
+        maven {
+            url "https://maven.google.com"
+        }
     }
 }

With this added, the project builds and installs, but then fails with a redbox error:

Default FirebaseApp is not initialized in this process com.multilogin. Make sure to call FirebaseApp.initializeApp(Context) first.
getInstance
    null:56
getInstance
    null
initialize
    RNPushNotificationModule.java:50
doInitialize
    ModuleHolder.java:198
markInitializable
    ModuleHolder.java:86
notifyJSInstanceInitialized
    NativeModuleRegistry.java:125
run
    CatalystInstanceImpl.java:387
handleCallback
    Handler.java:790
dispatchMessage
    Handler.java:99
dispatchMessage
    MessageQueueThreadHandler.java:31
loop
    Looper.java:164
run
    MessageQueueThreadImpl.java:194
run
    Thread.java:764

The docs say that manually calling FirebaseApp.initializeApp() is not required normally, that it will be done using information in google-services.json, but of course an out-of-box RN app won’t have one. Possibly the module could check if FirebaseApp.getApps(Context) is empty?

For now commenting out the new RNPushNotificationPackage() in MainApplication.java gets past this.

From there, cognito login fails with undefined is not an object (evaluating 'RNAWSCognito.computeModPow), since react-native link did not pick up the indirectly linked amazon-cognito-identity-js, so you have to react-native link amazon-cognito-identity-js too, or to have already added it as a direct NPM dependency before react-native link. Unfortunately, react-native link will add duplicate links for already linked packages, if you have already linked aws-amplify-react-native.

So in summary:

• Add docs for adding the google maven repo (or find an alternative, maybe including them in the repo?)
• Add docs for getting amazon-cognito-identity-js react-native linked
• Describe adding a google-services.json for firebase, and handle it not being initialized.