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 3 minutes and 50 seconds (queued for 1 second)
detached
Status Job ID Name Coverage
  Review
passed #591527
check_commit_log

00:00:22

passed #591529
eslint

00:01:03

passed #591528
js_check

00:00:35

 
  Build
passed #591530
build

00:01:46

 
  Test
passed #591531
test

00:00:42

passed #591532
test-pot

00:00:59