4 (4 ounce) boneless skinless chicken breast halves 3 tablespoons cornstarch 2 tablespoons soy sauce 1/2 teaspoon ground ginger 1/4 teaspoon garlic powder 3 tablespoons cooking oil, divided | 2 cups broccoli florets 1 cup sliced celery (1/2 inch pieces) 1 cup thinly sliced carrots 1 small onion, cut into wedges 1 cup water 1 teaspoon chicken bouillon granules |