applets/devicenotifier: properly set mount state of cd disks

Bohdan Onofriichukrequested to merge
littlesweet/plasma-workspace:cd_fix into master

Reason for the change

Such logic was before port from dataengine but on the qml side.

Test plan

Screenshots or screen recordings

Bugs fixed

BUG: 469706

Edited by Bohdan Onofriichuk

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