introduce ENVSYS_BATTERY_CHARGE, which is the same than an Indicator and
it's used to know if the battery is currently charging or discharging.
- Require two sensors at least for SME_CLASS_BATTERY to make the low-power
condition work: a ENVSYS_BATTERY_CAPACITY plus ENVSYS_BATTERY_CHARGE.
- Simplify sme_event_check_lowpower() and make it check the required
sensors in the SME_CLASS_ACADAPTER and SME_CLASS_BATTERY classes.
If the acadapter is not ready, trust the state returned by the battery
device.
Based on suggestion from joerg@.
4e10a84855