[Lock Screen] Do not try to unlock when unvisible

Summary:
In the lock screen, pressing the enter key while the UI is not yet
visible ends up trying to unlock (and would usually fail because the
password field is empty).
Ensure that the UI is visible before unlocking.

BUG: 395671

Test Plan:
Lock the session, press enter. The UI should appear without any
"Unlocking failed" message.

Reviewers: #plasma, #vdg, ngraham, davidedmundson

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

Subscribers: ngraham, davidedmundson, abetts, filipf, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D16219