android.os.SystemProperties 找不到符号

String SDK_DIR = System.getenv("ANDROID_SDK_HOME")
//("TAG", "SDK_DIR = " + SDK_DIR );
if(SDK_DIR == null) {
    Properties props = new Properties()
    props.load(new FileInputStream(project.rootProject.file("local.properties")))
    SDK_DIR = props.get('sdk.dir');
}
dependencies {
    provided files("${SDK_DIR}/platforms/android-21/data/layoutlib.jar")
}