Don't show an emblem for mounted disks, only unmounted disks

Summary:
The HIG now states that should only be used for non-default, uncommon, unusual, or unexpected statuses: https://hig.kde.org/components/assistance/emblem.html

Accordingly, we should only show an emblem when a disk is unmounted, not when it's mounted. Being mounted is the expected status for a disk; being unmounted-but-visible-anyway is what's unusual and deserves an emblem.

As an added bonus, this hugely improves the look of the Places panel by default.

Closes T9740

Test Plan:
Dolphin Places panel now:

Devices Notifier:

Reviewers: VDG, abetts

Reviewed By: VDG, abetts

Subscribers: broulik, kde-frameworks-devel

Tags: Frameworks

Maniphest Tasks: T9740

Differential Revision: https://phabricator.kde.org/D15935