online-accounts: Add shadow to providers icon

The new icons are flat and are more legible with a shadow, just
like the new GNOME application icons.

See Initiatives#2
11 jobs for add-shadow-to-icons in 3 minutes and 30 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #244280
build

00:02:24

 
  Test
passed #244282
asan

00:01:05

passed #244281
test

00:00:53

 
  Manual
manual #244287
allowed to fail manual
armv8
manual #244283
allowed to fail manual
flatpak
manual #244286
allowed to fail manual
msan
manual #244288
allowed to fail manual
ppc64le
manual #244284
allowed to fail manual
tsan
manual #244285
allowed to fail manual
ubsan
 
  Review
manual #244289
allowed to fail manual
review
manual #244290
allowed to fail manual
stop_review