On tablets with ABS_PRESSURE use a pressure value to determine tip state, not
BTN_TOUCH. This enables us (down the road) to have device-specific pressure
thresholds. For now we use a 5% default for all devices.

The threshold is a range, if we go past the upper range we initiate the tip
down, if we go below the lower range we release the tip again.

This affects two current tests:
* Once we have pressure offsets and pressure thresholds, we're biased towards
pressure. So we can only check that distance is zero when there is a pressure
value, not the other way round.
* When the pressure threshold is exceeded on proximity in with a nonzero
distance, we can only warn and handle the pressure as normal. Since this is a
niche case anyway anything fancier is likely unnecessary.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Acked-by: Jason Gerecke <jason.gerecke@wacom.com>