keep initialized state

i wanted to avoid needing to have the setup be an async function to
reduce code changes in the existing clients using the analytics
libraries, all of them are currently synchronous. Since the analytics
class stores an instance of each provider, adding the initialized flag
should work to prevent needing to call the Segment config method every
single time.

lib/flutter_app_analytics_segment_provider.dart

@@ -8,14 +8,20 @@ class SegmentProvider implements AnalyticsProvider {
   String _writeKey = '';
   String? _userId;
 
+  bool _initialized = false;
+
   SegmentProvider({
     required String writeKey,
     String? userId,
   }) {
     _writeKey = writeKey;
     _userId = userId;
   }
+
   initialize() async {
+    if (_initialized) {
+      return;
+    }
     await Segment.config(
       options: SegmentConfig(
         writeKey: _writeKey,
@@ -24,13 +30,15 @@ class SegmentProvider implements AnalyticsProvider {
         debug: false,
       ),
     );
+    _initialized = true;
   }
 
   @override
   Future<void> identify({
     String? userId,
     Map<String, dynamic>? properties,
   }) async {
+    initialize();
     if (userId != null && properties != null) {
       await Segment.identify(userId: _userId, traits: properties);
     } else if (userId != null) {
@@ -42,11 +50,13 @@ class SegmentProvider implements AnalyticsProvider {
 
   @override
   Future<void> trackEvent(AnalyticsEvent event) async {
+    initialize();
     await Segment.track(eventName: event.name, properties: event.properties);
   }
 
   @override
   Future<void> trackEvents(List<AnalyticsEvent> events) async {
+    initialize();
     await Future.forEach<AnalyticsEvent>(events, (event) => trackEvent(event));
   }
 }