[lockscreen] Clear password field when system suspends

Summary:
BUG: 414399
FIXED-IN: 5.18

Test Plan:

1. Lock screen
2. type something in the password field
3. Close lid/suspend system
4. Reopen Lid

Before:
Password text is kept as in step 2

After:
Password text is empty

Reviewers: Plasma, broulik, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D25487