FlutterEngineGroup memory test: added precache before executing gradle. (#75793)

dev/devicelab/bin/tasks/flutter_engine_group_performance.dart

@@ -59,6 +59,8 @@ Future<TaskResult> _doTest() async {
     final String gradlewExecutable =
         Platform.isWindows ? '.\\$gradlew' : './$gradlew';
     final String flutterPath = path.join(flutterDirectory, 'bin', 'flutter');
+    await utils.eval(flutterPath, <String>['precache', '--android'],
+        workingDirectory: modulePath);
     await utils.eval(flutterPath, <String>['pub', 'get'],
         workingDirectory: modulePath);
     _copyGradleFromModule(modulePath, androidPath);