Confusing behavior when long intervals between .`update()`

When there is a long interval between .`update()` calls, `.value` is not what is expected.

Originally from https://discord.com/channels/327254708534116352/537365702651150357/1341836635373047941

```py
import board, digitalio, time
from adafruit_debouncer import Button

# Tested on a Circuit Playground Bluefruit, which connects the far side of the buttons to +V.
Adjust pulls and value_when_pressed as needed.

button_A_input = digitalio.DigitalInOut(board.BUTTON_A)
button_A_input.switch_to_input(digitalio.Pull.DOWN)
button_A = Button(button_A_input, value_when_pressed = True)
button_B_input = digitalio.DigitalInOut(board.BUTTON_B)
button_B_input.switch_to_input(digitalio.Pull.DOWN)
button_B = Button(button_B_input, value_when_pressed = True)

def step(t, msg):
button_A.update()
button_B.update()
print(f"time: {t}, button_A: {button_A.pressed}, button_B: {button_B.pressed}")
print(msg)
time.sleep(5)
return t + 5

t = 0
t = step(t, "start with no buttons pressed")
t = step(t, "continue with no buttons pressed")
t = step(t, "press and hold A")
t = step(t, "continue holding A")
t = step(t, "press and hold B, in addition")
t = step(t, "continue holding A and B")
t = step(t, "continue holding A and B")
t = step(t, "release A")
t = step(t, "release B")
t = step(t, "continue with both released")
```
Follow the directions as printed when testing.
```
time: 0, button_A: False, button_B: False
start with no buttons pressed
time: 5, button_A: False, button_B: False
continue with no buttons pressed
time: 10, button_A: False, button_B: False
press and hold A
time: 15, button_A: False, button_B: False
continue holding A
time: 20, button_A: True, button_B: False
press and hold B, in addition
time: 25, button_A: False, button_B: False
continue holding A and B
time: 30, button_A: False, button_B: True
continue holding A and B
time: 35, button_A: False, button_B: False
release A
time: 40, button_A: False, button_B: False
release B
time: 45, button_A: False, button_B: False
continue with both released
```

Start by reproducing the provided CircuitPython example with Button.update() and inspect the Button implementation, focusing on how .pressed is computed after each five-second sleep. Compare each printed state with the physical actions and establish the intended behavior for long gaps before adding a regression test and confirming the corrected sequence.

Written by the indexing model from the issue text.