There are two main changes in this commit:
* Listen to the `prepare-for-sleep` signal from `LoginManager`, which
is emitted just before suspending and just after resuming. When the
signal is received, update the user’s screen time state (active or
inactive), add a transition if necessary, and save the screen time
history if necessary.
* Factor the `preparingForSleep` property of `LoginManager` into the
user’s screen time state, meaning that the user will be considered
inactive between the system going for suspend and coming back from
resume.
The rest of the changes in the commit are boilerplate to allow for this
functionality to be unit tested.
Signed-off-by: Philip Withnall <pwithnall@gnome.org>
Closes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/8185
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3643>
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