unlockDialog: Use "normal" background

Now that we apply a strong blur effect to the background, it doesn't
make too much sense to use a separate lock-screen background: It will
be mostly unrecognizable anyway.

The alternative would be to turn off the blur effect if a different
background is used (or have a hidden setting for that), but that would
then imply that we must keep the contents readable without blur.

Let's avoid that rabbit hole and just re-use the regular background.

GNOME/gnome-shell!1001
6 jobs for !1001 with normal-unlock-bg in 8 minutes and 40 seconds
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Status Job ID Name Coverage
  Review
passed #603181
check_commit_log

00:00:28

passed #603183
eslint

00:01:11

passed #603182
js_check

00:00:33

 
  Build
passed #603184
build

00:05:56

 
  Test
passed #603185
test

00:01:18

passed #603186
test-pot

00:01:31