Bug Fixes:
B20180001: Can not set the sound in the external storage to the notification sound of reminder. (Occurred on < Android 6.0)
B20180002: An exception may occur when deleting a task list.
B20180003: An exception occurs when you refuse with a permission request. (Occurred on Android 6.0)
B20180004: NullPointerException occurs when In-app billing service can not be set up.
