Await analytics initialization on client creation

[agalambas]

/packages/core/lib/client.dart

+ Future<Analytics> createClient(Configuration configuration) async {
- Analytics createClient(Configuration configuration) {
    if (configuration.debug) {
      Analytics.debug = true;
    }
    if (configuration.flushPolicies == null) {
      configuration = setFlushPolicies(configuration, defaultFlushPolicies);
    }

    final analytics = Analytics(configuration, storeFactory());

    if (configuration.debug) {
      analytics.addPlugin(EventLogger());
    }

+.  await analytics.init();
-   analytics.init();
    ScreenObserver().screenStream.listen((name) {
      analytics.screen(name);
    });

    return analytics;
  }