autostart: Replace should_show() with get_show_in() again

GDesktopAppInfo's should_show() implementation also considers the
NoDisplay flag, which is generally used by core components like
window managers. Return to use get_show_in(), which just checks
ShowOnlyIn and friends based on XDG_CURRENT_DESKTOP to get the
session back working.

https://bugzilla.gnome.org/show_bug.cgi?id=729575