mv FirebaseInstanceId usage out of getToken

* Move the implementation of getToken into it's own method.
   - Allows us to add another implementation in a follow up commit

Author: jkasten2

OneSignalSDK/onesignal/src/main/java/com/onesignal/PushRegistratorFCM.java

@@ -57,6 +57,10 @@ String getProviderName() {
    @Override
    String getToken(String senderId) throws Throwable {
       initFirebaseApp(senderId);
+      return getTokenWithClassFirebaseInstanceId(senderId);
+   }
+
+   private String getTokenWithClassFirebaseInstanceId(String senderId) throws IOException {
       FirebaseInstanceId instanceId = FirebaseInstanceId.getInstance(firebaseApp);
       return instanceId.getToken(senderId, FirebaseMessaging.INSTANCE_ID_SCOPE);
    }