altos: Use simpler debounce logic for buttons

altos: Use simpler debounce logic for buttons

Instead of waiting for a while after the transition to decide if it
has stuck, signal the event right away and then ignore other
transitions for the debounce interval.

This seems to work just as reliably, but has the benefit of
eliminating button latency at press time.

Signed-off-by: Keith Packard <keithp@keithp.com>