adc_battery: standardize sysfs battery paths.

-sysfs base path:
/sys/devices/platform/sec-battery/power_supply/battery
or
/sys/class/power_supply/battery

-healthd expects "current_now"
See: https://github.com/Team-Exhibit/beanstalk_system_core/blob/kk44/healthd/BatteryMonitor.cpp#L353

-healthd expects "voltage_now" or "batt_vol" but voltage_now
also enables Settings -> performance -> battery in
user space for ROM's that use the opensource performance control.
See: https://github.com/Team-Exhibit/beanstalk_system_core/blob/kk44/healthd/BatteryMonitor.cpp#L338
See also: https://github.com/omnirom/android_packages_apps_PerformanceControl/blob/android-4.4/src/com/brewcrewfoo/performance/util/Constants.java#L79