service: Add helper function for detecting a default service index

Add a helper function to detect if the index belongs to the default
service.

src/connman.h

@@ -638,6 +638,7 @@ struct connman_service *__connman_service_lookup_from_index(int index);
 struct connman_service *__connman_service_lookup_from_ident(const char *identifier);
 struct connman_service *__connman_service_create_from_network(struct connman_network *network);
 struct connman_service *__connman_service_create_from_provider(struct connman_provider *provider);
+bool __connman_service_index_is_default(int index);
 struct connman_service *__connman_service_get_default(void);
 void __connman_service_update_from_network(struct connman_network *network);
 void __connman_service_remove_from_network(struct connman_network *network);

src/service.c

@@ -1365,6 +1365,18 @@ struct connman_service *__connman_service_get_default(void)
 	return service;
 }
 
+bool __connman_service_index_is_default(int index)
+{
+	struct connman_service *service;
+
+	if (index < 0)
+		return false;
+
+	service = __connman_service_get_default();
+
+	return __connman_service_get_index(service) == index;
+}
+
 static void default_changed(void)
 {
 	struct connman_service *service = __connman_service_get_default();