chargethresholdhelper: Filter out batteries without thresholds early

Previously, getBatteries() returned all batteries, even those without support
for thresholds. While getthreshold simply skipped those where reading
fails, setthreshold exited early on the first failure, leaving the remaining
batteries untouched.

By already performing filtering in getBatteries(), this should now behave more
consistently and work also if not all batteries support thresholds.

BUG: 464535