code: public void init(final Context context, final Handler mainThreadHandler, final Logger logger, final PushTokenProvider provider, final PushTokenListener tokenListener, final PushMessageListener messageListener) {
          log = logger;
          this.provider = provider != null ? provider : () -> FirebaseInstanceId.getInstance().getToken();
          registerPushReceiver(mainThreadHandler, context, this.provider, tokenListener, messageListener);
      }
docs: Initialise PushManager.
  
  @param context           Application context.
  @param mainThreadHandler Main thread handler to post push messages.
  @param logger            Internal logger.
  @param provider          Provides fcm push token.
  @param tokenListener     Listener for refreshed push tokens.
  @param messageListener   Listener for push messages.