HID: 3m: Correct touchscreen emulation

The current code sometimes misses to report the last BTN_TOUCH event
when multiple fingers are lifted simultaneously.  With the
introduction of MT slots, the tracking id is available to determine
the oldest active contact. Use this information to simplify and
correct the touchscreen emulation logic.

Signed-off-by: Henrik Rydberg <rydberg@euromail.se>
Acked-by: Stéphane Chatty <chatty@enac.fr>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>