open-uri: Use correct app ID to determine whether it exists

x-d-p represents apps by their app ID *without* the .desktop suffix,
but GDesktopAppInfo represents them by the app ID *with* the .desktop
suffix.

Fixes: 69205f12 "open-uri: Show app chooser when default app does not exist"
Resolves: #524
Signed-off-by: Simon McVittie <smcv@collabora.com>