Commit 17380886 in the devicenotifications data engine brought in
two problems with the Device Notifier:
- the notifications are now cleared once a device is unmounted,
this results in the "You can now safely remove this device"
message not to be shown anymore.
- since each notification is no longer its own data source, it
is possible to get into a situation where one device would only
receive a first notification but not any subsequent ones.
This patch attempts to fix these two problems.