dnd: Immediately start drag done by pointer devices
Unlike touch screens, pointer devices (e.g. cursor, touchpad) are
basically never used to switch app grid pages, which is the main
use case for the drag threshold. This point was recently raised
as design feedback.
Immediately start drags when using pointer devices.
See https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/3802#note_1124701