Hides configuration from public interface and make isBuild final

- The purpose of these changes is because having both configuration and isBuilt
publicly mutable is misleading, since changing those attributes will have no impact.

Author: gabrielgarciagava

lib/amplitude.dart

@@ -10,7 +10,7 @@ import 'events/base_event.dart';
 import 'events/group_identify_event.dart';
 
 class Amplitude {
-  Configuration configuration;
+  final Configuration _configuration;
   MethodChannel _channel = const MethodChannel('amplitude_flutter');
 
   /// Whether the Amplitude instance has been successfully initialized
@@ -20,7 +20,7 @@ class Amplitude {
   /// // If care about init complete
   /// await amplitude.isBuilt;
   /// ```
-  late Future<bool> isBuilt;
+  late final Future<bool> isBuilt;
 
   /// Returns an Amplitude instance
   ///
@@ -29,15 +29,16 @@ class Amplitude {
   /// // If care about init complete
   /// await amplitude.isBuilt;
   /// ```
-  Amplitude(this.configuration, [MethodChannel? methodChannel]) {
+  Amplitude(Configuration configuration, [MethodChannel? methodChannel])
+      : _configuration = configuration {
     _channel = methodChannel ?? _channel;
     isBuilt = _init();
   }
 
   /// Private method to initialize and return a `Future<bool>`
   Future<bool> _init() async {
     try {
-      await _channel.invokeMethod('init', configuration.toMap());
+      await _channel.invokeMethod('init', _configuration.toMap());
       return true; // Initialization successful
     } catch (e) {
       print('Error initializing Amplitude: $e');