plasma-shutdown: Correclty detect if we're using the legacy session

We need different shutdown paths for the systemd and legacy boot. The
current code worked by making the systemd call to end the session and
checking the return status.

This worked for handling the situation where systemd is not installed.
It does not capture the case where the systemd boot is explicitly
disabled. The call successfully does nothing.

Instead replace with an explicit check that our target is activated.

Also change to end the plasma-workspace.target as that's semantically
more correct given it's the one we started. This is bound to the
graphical-session so it ends up being the same thing.

BUG: 483066


(cherry picked from commit 41629e86)