fix(demo): make Android channel ID configurable

Author: fadi-george

examples/demo/.env.example

@@ -2,3 +2,7 @@
 ONESIGNAL_APP_ID=your-onesignal-app-id
 ONESIGNAL_API_KEY=your_rest_api_key
 E2E_MODE=false
+
+# Optional: Android Notification Channel ID for the WITH SOUND test notification.
+# Create one in your OneSignal dashboard under Settings > Android Notification Categories.
+ONESIGNAL_ANDROID_CHANNEL_ID=

examples/demo/Assets/Scripts/Services/OneSignalApiService.cs

@@ -16,6 +16,8 @@ public class OneSignalApiService
 
         private const string PlaceholderApiKey = "your_rest_api_key";
 
+        private const string DefaultAndroidChannelId = "b3b015d9-c050-4042-8548-dcc34aa44aa4";
+
         public void SetAppId(string appId) => _appId = appId;
 
         public string GetAppId() => _appId;
@@ -28,6 +30,12 @@ public bool HasApiKey()
 
         private static string GetApiKey() => DotEnv.Get("ONESIGNAL_API_KEY");
 
+        private static string GetAndroidChannelId()
+        {
+            var value = DotEnv.Get("ONESIGNAL_ANDROID_CHANNEL_ID")?.Trim();
+            return string.IsNullOrEmpty(value) ? DefaultAndroidChannelId : value;
+        }
+
         public async Task<bool> SendNotification(NotificationType type, string subscriptionId)
         {
             if (string.IsNullOrEmpty(subscriptionId) || string.IsNullOrEmpty(_appId))
@@ -59,7 +67,7 @@ public async Task<bool> SendNotification(NotificationType type, string subscript
                     extra = new JObject
                     {
                         ["ios_sound"] = "vine_boom.wav",
-                        ["android_channel_id"] = "b3b015d9-c050-4042-8548-dcc34aa44aa4",
+                        ["android_channel_id"] = GetAndroidChannelId(),
                     };
                     break;
                 default: