[Fstab] Use folder-decrypted icon for encrypting fuse mounts

Summary:
When Vaults are mounted and show up in the mtab, use the matching icon.
As these mounts are only visible in the decrypted state, use the
folder-decrypted icon.

Test Plan: Vault shows up with matching icon

Reviewers: Frameworks, ngraham, hallas

Reviewed By: ngraham

Subscribers: broulik, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D21041