evdev: get the current slot value from the absinfo struct or mtdev

evdev: get the current slot value from the absinfo struct or mtdev

If touches are already present on the device, absinfo has the currently
active touch slot. There's a race condition where the slot may change before
we enable the fd and we thus miss out on the ABS_MT_SLOT event. It's still
slightly more correct than assuming whatever comes next is slot 0.