使用下面的代码，获取安卓内置或外置SD卡根目录：

//内置sd卡路径
String sdcardPath = System.getenv("EXTERNAL_STORAGE");
//内置sd卡路径
String sdcardPath = Environment.getExternalStorageDirectory().getAbsolutePath();

//外置sd卡路径
String extSdcardPath = System.getenv("SECONDARY_STORAGE");