fallback: add timer-based touch arbitration

When a hand is resting on a pen+touch device, lifting the hand may remove the
stylus from proximity before the hand leaves the surface. If the kernel
performs touch arbitration, this triggers a touch down on proximity out,
followed by a touch up immediately after when the hand stops touching.

This can cause ghost touch events. Prevent this by using a timer-based
arbitration toggle.

Same as 2a378bea but for the fallback
interface.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>