loginManager: Use signal argument to detect preparing for sleep

loginManager: Use signal argument to detect preparing for sleep

The DBus PreparingForSleep property on org.freedesktop.login1.Manager
does not trigger PropertyChanged signals, leading to wrong values due to
gdbus caching. In most cases it would have always been false.

Additionally it was not included in the XML interface description file
included in gnome-shell. So it was actually undefined.

Since this property is used in _calculateUserStateFromLogind() to
determine that a user is not active when closing the lid on a laptop,
the user was considered still active.

Fix this by storing the "start" argument from the PrepareForSleep signal
instead of trying to read from the property.

Fixes: 6a43b6f55 ("timeLimitsManager: Store screen time state on suspend/resume")
Closes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/8185
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3679>