Close screen grabbing effect when screensaver starts

Summary:
The screenlock fails on X11 if it can't grab the keyboard.

We can't nicely solve the generic case. We can solve the common case of
a kwin effect being active.

It's not critical, arguably not even desirable to have these effects
persist after the screen is locked through an external trigger. We can
just close the effect early.

Key grabs have to be relased early before the close animation completes
so that the locker doesn't have a race based on animation times.

It's not ideal, but no worse than the current state for not much work.

BUG: 234153

Test Plan:
locked screen on a timer
opened various effects

Reviewers: #kwin, zzag

Reviewed By: #kwin, zzag

Subscribers: ngraham, zzag, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D20890