[crashlytics] Add first version of crashlytics functionality

tests/android/app/build.gradle

@@ -1,5 +1,6 @@
 apply plugin: "com.android.application"
 apply plugin: "com.google.firebase.firebase-perf"
+apply plugin: 'io.fabric'
 
 import com.android.build.OutputFile
 
@@ -93,6 +94,9 @@ dependencies {
     compile "com.google.firebase:firebase-storage:$firebaseVersion"
     compile "com.google.firebase:firebase-firestore:$firebaseVersion"
     compile "com.google.firebase:firebase-invites:$firebaseVersion"
+    compile('com.crashlytics.sdk.android:crashlytics:2.7.1@aar') {
+        transitive = true
+    }
     compile "com.android.support:appcompat-v7:26.0.1"
     compile "com.facebook.react:react-native:+"  // From node_modules
 }

tests/android/app/src/main/java/com/reactnativefirebasedemo/MainApplication.java

@@ -10,6 +10,7 @@ import io.invertase.firebase.auth.RNFirebaseAuthPackage;
 import io.invertase.firebase.config.RNFirebaseRemoteConfigPackage;
 import io.invertase.firebase.crash.RNFirebaseCrashPackage;
 import io.invertase.firebase.database.RNFirebaseDatabasePackage;
+import io.invertase.firebase.fabric.crashlytics.RNFirebaseCrashlyticsPackage;
 import io.invertase.firebase.firestore.RNFirebaseFirestorePackage;
 import io.invertase.firebase.links.RNFirebaseLinksPackage;
 import io.invertase.firebase.messaging.RNFirebaseMessagingPackage;
@@ -43,6 +44,7 @@ public class MainApplication extends Application implements ReactApplication {
           new RNFirebaseAuthPackage(),
           new RNFirebaseRemoteConfigPackage(),
           new RNFirebaseCrashPackage(),
+          new RNFirebaseCrashlyticsPackage(),
           new RNFirebaseDatabasePackage(),
           new RNFirebaseFirestorePackage(),
           new RNFirebaseLinksPackage(),