power: Lock screen when the lid is closed without external monitors

We currently only lock the screen when suspend is disabled on the
session level. This is somewhat ironic, because in reality suspend
happens because of systemd-logind.

In the case where we suspend, it does not hurt to also explicitly lock
the screen. In the case that suspend is inhibited (through GNOME
Tweaks), this fixes issues where the session remains unlocked.

See also #88
and gnome-tweaks!33
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