Turn off External Drives' notification,remove the SD card and reinsert it, notification still pop up.

[Hannah Zhu]

Information:
phablet@ubuntu-phablet:~$ system-image-cli -i
current build number: 402
device name: krillin
channel: ubuntu-touch/rc-proposed/bq-aquaris.en
last update: 2016-08-08 09:35:04
version version: 402
version ubuntu: 20160806
version device: 20160606-ab415b2
version custom: 20160701-981-38-14

Precondition:
SD card is inserted in the device.

Steps:
1. Open System Settings
2. Tap on Notifications
3. Turn off External Drives
4. Remove the SD card and reinsert it

Actual result:
Notification of External Device still pop up

Expected result:
Turn off External Drives' notification,remove the SD card and reinsert it, notification should not pop up.