cleanup: Minimize deep_unpack() usage

cleanup: Minimize deep_unpack() usage

deep_unpack()[1] will unpack a variant and its children, but only up to one level.
lookup_value()[2] will directly search for a value with a linear scan.

Performing a deep_unpack() + lookup can be a more expensive operation when we are
looking for just a single value compared to just perform the lookup_value() directly
in C. Avoid the deep_unpack() usage when we perform a single check presence.

[1] https://gjs.guide/guides/glib/gvariant.html#deepunpack
[2] https://docs.gtk.org/glib/method.Variant.lookup_value.html

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2416>