sleep: allow HibernateDelaySec and low-battery hibernation to work together

sleep: allow HibernateDelaySec and low-battery hibernation to work together

Previously, setting HibernateDelaySec= would disable ACPI battery trip
point (_BTP) alarms, forcing the system to rely solely on software
polling for battery checks. This could result in the battery draining
to 0% between polling intervals, causing data loss.

Now, when ACPI _BTP is available AND HibernateDelaySec= is set, both
mechanisms work together. The system will hibernate on whichever comes
first: low battery (instant hardware alarm) or the configured timeout.

This also properly respects HibernateOnACPower=no by resetting the
timer while on AC power, matching the documented behavior.

Fixes: https://github.com/systemd/systemd/issues/26498