FireflyApi2 外部存储

1. 获取外部存储U盘路径

   函数：public String getUSBPath(int num)
   描述：获取外部存储U盘路径
   参数：num u盘的index
   返回：失败返回null
   范例：
   String usb1_path  = mFireflyApi.getUSBPath(1);

2. 获取外部存储SD卡路径

   函数：public String getSDcardPath()
   描述：获取外部存储SD卡路径
   返回：失败返回null
   范例：
   String sd_path  = mFireflyApi.getSDcardPath();

3. 卸载外部存储

   函数：public　void doUnmountVolume(String path,boolean force,boolean removeEncryption)
   描述：卸载外部存储
   参数：path 路径为volume mountPoint，负责会抛错
   　　　force　强制卸载,为false时被占用的设备不会卸载
      　removeEncryption　为ture时，包含了force为true,此时会强制卸载并移除加密映射
   /*
    * If force is not set, we do not unmount if there are
    * processes holding references to the volume about to be unmounted.
    * If force is set, all the processes holding references need to be
    * killed via the ActivityManager before actually unmounting the volume.
    * This might even take a while and might be retried after timed delays
    * to make sure we dont end up in an instable state and kill some core
    * processes.
    * If removeEncryption is set, force is implied, and the system will remove any encryption
    * mapping set on the volume when unmounting.
    */
   备注：调用此接口卸载外部存储时，先用StorageList.getVolumeState来检测path的挂载状态，为Environment.MEDIA_MOUNTED时再卸载
   范例：
   StorageList mStorageList = new StorageList(context);
   if(Environment.MEDIA_MOUNTED.equals(mStorageList.getVolumeState(path)))
   	mFireflyApi.doUnmountVolume("/mnt/sdcard/",true,false);

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