appdata: Use the current app id

This is to fix flatpak build-bundle: Invalid id gtranslator during build.

The problematic part of !6 (merged) in a separate MR. In theory it should work fine. In practice:

$ flatpak-builder --finish-only app ${MANIFEST_PATH}
Cleaning up
Renaming org.gnome.Gtranslator.appdata.xml to share/appdata/org.gnome.Gtranslator.appdata.xml
Rewriting contents of org.gnome.Gtranslator.desktop
Running appstream-compose
Processing application org.gnome.Gtranslator
Error loading desktop file: Failed to find icon: Failed to find icon org.gnome.Gtranslator
Error: ERROR: appstream-compose failed: Child process exited with code 1
ERROR: Job failed: exit code 1