appinfo: Launch the OpenURI portal using a synchronous D-Bus call

Calling the D-Bus method for the OpenURI portal "protects" the logic from
not ever having the remote method running in case the xdg-desktop-portal
process is not yet running and the caller quits quickly after the call.

This should not be a problem as the method returns immediately (regardless
of the user making a selection), but making it synchronous would prevent
situations where the OpenURI method would never be called because of D-Bus
dropping the message after the caller dies, without explicitly waiting for
a reply.

https://bugzilla.gnome.org/show_bug.cgi?id=780441