Detection logic is refactored - battery voltage delta is tested each
50ms, voltage is considered stable when difference of last 10 samples
is smaller than 100mV
This makes stable threshold more forgiving than betaflight#13350 (time for
comparison is 50 instead of 500ms).

- voltageStablePrevFiltered every time delta is exceeded
- voltage within range is ANDed over 100ms periods
- voltage is stable if it was within range for 10 out of 11 periods
  - slowly changing voltage will update threshold, but voltage will be
    considered stable
  - 1 update/s (100mV/s) is tolerated