drivers: input: input_adc_keys: add debouncing to avoid false events

The previous implementation already reported a key-press event after
reading an ADC value close to the threshold for the first time.

This may lead to false events if the ADC takes a reading just during
the transition from one button state to another (especially if a
somewhat large capacitor is used to avoid noise).

A key-press state must be same for at least two samples in order to
avoid such issues, which is what this commit implements.

Signed-off-by: Martin Jäger <martin@libre.solar>