For devices that are incapable of reporting per-contact distance and can
only report ABS_DISTANCE, just distance is not enough for upper layers of
OS to determine whether contact is leaving the area or if it is continuing
hovering, we need BTN_TOOL_FINGER for that:

                                   Contact State
                    Inactive          Hovering            Active
                   (far away)    (inside hover area)  (touching surface)
BTN_TOUCH              0                 0                   1
BTN_TOOL_FINGER        0                 1                   1
ABS_DISTANCE          N/A                1                   0
Signed-off-by: KT Liao <kt.liao@emc.com.tw>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>