[in_app_purchase_android] Add missing response code to BillingResponse enum (flutter#5120)

This response code was missing. 

Fixes flutter#135540

Author: gmackall

packages/in_app_purchase/in_app_purchase_android/CHANGELOG.md

@@ -1,3 +1,7 @@
+## 0.3.0+15
+
+* Adds missing network error response code to BillingResponse enum.
+
 ## 0.3.0+14
 
 * Updates annotations lib to 1.7.0.

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

@@ -414,6 +414,10 @@ enum BillingResponse {
   /// Failure to consume since item is not owned.
   @JsonValue(8)
   itemNotOwned,
+
+  /// Network connection failure between the device and Play systems.
+  @JsonValue(12)
+  networkError,
 }
 
 /// Serializer for [BillingResponse].

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

@@ -2,7 +2,7 @@ name: in_app_purchase_android
 description: An implementation for the Android platform of the Flutter `in_app_purchase` plugin. This uses the Android BillingClient APIs.
 repository: https://github.com/flutter/packages/tree/main/packages/in_app_purchase/in_app_purchase_android
 issue_tracker: https://github.com/flutter/flutter/issues?q=is%3Aissue+is%3Aopen+label%3A%22p%3A+in_app_purchase%22
-version: 0.3.0+14
+version: 0.3.0+15
 
 environment:
   sdk: ">=2.19.0 <4.0.0"

packages/in_app_purchase/in_app_purchase_android/pubspec.yaml

@@ -54,6 +54,7 @@ void main() {
     converter.fromJson(6);
     converter.fromJson(7);
     converter.fromJson(8);
+    converter.fromJson(12);
   });
 
   group('startConnection', () {