[Device Notifier] Only ever pop up if device is visible in the list

When a device notification arrives, such as "you can now safely unmount" or "failed to unmount"
device notifier would unconditionally open showing an empty list.

Since we now show device notifications only connected to the device they're about, it makes no
sense to expand it for invisible devices. Applications where mounting or unmounting can fail
for non-removable devices, like Dolphin, already show the error message within the UI anyway,
making the popup redundant.

BUG: 368894
FIXED-IN: 5.8.0

Differential Revision: https://phabricator.kde.org/D2823