user-accounts: Use a more specific GtkSettings object

Otherwise we might potentially be using the GtkSettings for the wrong
screen, which might make a difference if one screen has animations
enabled but another doesn’t.

(This is an edge case I spotted while reading the code, but not one that
I’ve experienced practically.)

Signed-off-by: Philip Withnall <withnall@endlessm.com>
11 jobs from carousel-arrow in 9 minutes and 5 seconds (queued for 3 seconds)
latest
Status Job ID Name Coverage
  Build
passed #190657
build

00:04:00

 
  Test
passed #190659
asan

00:05:05

passed #190658
test

00:04:48

 
  Manual
manual #190664
allowed to fail manual
armv8
manual #190660
allowed to fail manual
flatpak
manual #190663
allowed to fail manual
msan
manual #190665
allowed to fail manual
ppc64le
manual #190661
allowed to fail manual
tsan
manual #190662
allowed to fail manual
ubsan
 
  Review
manual #190666
allowed to fail manual
review
manual #190667
allowed to fail manual
stop_review