[Device Notifier] Restore busy indicator

Summary:
This patch fixes two issues:
- the 'state' of the device now propagates correctly, that makes the busy indicator be displayed again, device emblems are also updated correctly
- there is no more delay between the moment when the device is actually mounted and the moment the device notifier says it is. The worst case used to be 5 seconds.

BUG: 354321
BUG: 399986

Reviewers: #plasma, #vdg, broulik, bruns, ngraham, davidedmundson

Reviewed By: #plasma, #vdg, ngraham, davidedmundson

Subscribers: mart, cfeck, ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D16170