on-ac-power: rework logic

on-ac-power: rework logic

History of the function:
96788d2aa4f4b0b49874b4a240ce47d9e8485d1b assume system is running on AC power when no battery found
795e86b4f1e8a1fd440f8c817621779c6aedbdb5 ignore USB-C ports in power source mode when detecting system is running on AC power
c19a51bec40ae5e5073464e72411e7920d05d683 invert ac_power() source type check
6d89003462484c8656b698e07b9cf0a337e3818e assume ac when /sys/class/power_supply is missing
240dbaa44f8e5ad51775c776fc3ce9cd2f19f037 add ConditionACPower=

Interestingly, the return condition 'on_ac_power == found_online || !found_offline'
was there from the very beginning, and even Yu's latest change doesn't change this,
but only extends it to 'on_ac_power == found_online || !found_offline || !found_battery'.
This means that any system with no AC power supply will be unconditionally classified
as on_ac_power.

Let's change the logic: if we have an online AC supply, answer is "yes". If no
supplies, but we have a battery, answer is "no". Otherwise, assume "yes", based
on the assumption that presense of a battery would at least be always reported,
even if an AC power supply might not be.

Fixes #24407. It also shouldn't impact previous fixes: assume ac when
/sys/class/power_supply is missing, ignore USB-C ports in power source mode,
assume system is running on AC power when no battery found.