evdev: Always create the main seat

There could be times when we may not necessarily see a device appear
at initialization time, like when we're VT switched away when we
initialize, and thus we can't ever rely on a main seat appearing.

Always create a main seat with logical pointer/keyboard devices, and
tie the first physical seat that comes in to the main seat.

https://bugzilla.gnome.org/show_bug.cgi?id=726199