skip 'generateLockfiles' for add-to-app project. (#91458)

packages/flutter_tools/gradle/flutter.gradle

@@ -137,6 +137,7 @@ class FlutterPlugin implements Plugin<Project> {
         this.project = project
 
         def rootProject = project.rootProject
+        if (isFlutterAppProject()) {
             rootProject.tasks.register('generateLockfiles') {
                 rootProject.subprojects.each { subproject ->
                     def gradlew = (OperatingSystem.current().isWindows()) ?
@@ -148,6 +149,7 @@ class FlutterPlugin implements Plugin<Project> {
                     }
                 }
             }
+        }
 
         // Configure the Maven repository.
         String hostedRepository = System.env.FLUTTER_STORAGE_BASE_URL ?: DEFAULT_MAVEN_HOST