Write documentation and remove todos, remove unused import

packages/in_app_purchase/in_app_purchase_android/lib/src/billing_client_wrappers/billing_client_wrapper.dart

@@ -325,12 +325,15 @@ class BillingClient {
     return result ?? false;
   }
 
-  /// TODO
+  /// BillingConfig method channel string identifier.
+  //
+  // Must match the value of GET_BILLING_CONFIG in
+  // ../../../android/src/main/java/io/flutter/plugins/inapppurchase/MethodCallHandlerImpl.java
   @visibleForTesting
   final String getBillingConfigMethodString =
       'BillingClient#getBillingConfig()';
 
-  /// TODO
+  /// Fetches billing config info into a [BillingConfigWrapper] object.
   Future<BillingConfigWrapper> getBillingConfig() async {
     return BillingConfigWrapper.fromJson((await channel
             .invokeMapMethod<String, dynamic>(getBillingConfigMethodString)) ??

packages/in_app_purchase/in_app_purchase_android/lib/src/billing_client_wrappers/billing_config_wrapper.dart

@@ -25,7 +25,7 @@ const String kInvalidBillingConfigErrorMessage =
 @immutable
 class BillingConfigWrapper implements HasBillingResponse {
   /// Constructs the object with [responseCode] and [debugMessage].
-  const BillingConfigWrapper({required this.responseCode, this.debugMessage, this.countryCode});
+  const BillingConfigWrapper({required this.responseCode, this.debugMessage, this.countryCode = ''});
 
   /// Constructs an instance of this from a key value map of data.
   ///
@@ -55,7 +55,7 @@ class BillingConfigWrapper implements HasBillingResponse {
 
   /// https://developer.android.com/reference/com/android/billingclient/api/BillingConfig#getCountryCode()
   @JsonKey(defaultValue: '')
-  final String? countryCode;
+  final String countryCode;
 
   @override
   bool operator ==(Object other) {

packages/in_app_purchase/in_app_purchase_android/lib/src/in_app_purchase_android_platform_addition.dart

@@ -148,10 +148,13 @@ class InAppPurchaseAndroidPlatformAddition
     );
   }
 
-  /// TODO
-  Future<BillingConfigWrapper> getBillingConfig() async {
+  /// Returns Play billing country code based on ISO-3166-1 alpha2 format.
+  ///
+  /// See: https://developer.android.com/reference/com/android/billingclient/api/BillingConfig
+  /// See: https://unicode.org/cldr/charts/latest/supplemental/territory_containment_un_m_49.html
+  Future<String> getBillingConfig() async {
     return _billingClientManager.runWithClientNonRetryable(
-      (BillingClient client) => client.getBillingConfig(),
+      (BillingClient client) async => (await client.getBillingConfig()).countryCode,
     );
   }
 }

packages/in_app_purchase/in_app_purchase_android/test/billing_client_wrappers/purchase_wrapper_test.dart

@@ -4,7 +4,6 @@
 
 import 'package:in_app_purchase_android/billing_client_wrappers.dart';
 import 'package:in_app_purchase_android/in_app_purchase_android.dart';
-import 'package:in_app_purchase_android/src/billing_client_wrappers/billing_config_wrapper.dart';
 import 'package:test/test.dart';
 
 const PurchaseWrapper dummyPurchase = PurchaseWrapper(