appdata: Keep the same app ID as in 0.13

Go back to using "org.gnome.Geary.desktop" as it was in 0.13. The
appdata spec is very clear that the app ID is not supposed to change and
should be a unique identifier for the app across distributions.
5 jobs for wip/kalev/appdata-id in 27 minutes and 52 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Build
passed #270272
fedora

00:09:39

passed #270273
ubuntu

00:12:22

 
  Test
passed #270274
flatpak

00:15:03

 
  Deploy
passed #270275
review

00:00:25

manual #270276
allowed to fail manual
stop_review