device-manager-evdev: Set and unset the stage for the main seat too

device-manager-evdev: Set and unset the stage for the main seat too

When no input devices are available on startup the device manager might be fast
enough to be constructed so that no default stage is set yet, and thus when
main seat virtual devices are created they won't have a proper stage set.
If then we plug a real device, the events that an input manager could generate
won't be associated to any stage and thus won't be processed.

We need then ensure that when we update the stage for the device manager we
(un)associate it also to the main seat devices.

(cherry picked from commit d7bdc1591fba5353b23a28ebf676da0e7b58c84e)

Origin: upstream, 3.28.3, commit:72965aaaf0b1c6cf098a0652e14dd3eb4c1501b8

Gbp-Pq: Name device-manager-evdev-Set-and-unset-the-stage-for-the-main.patch