Where is the current_now measurement generated
Basically there are battery drivers in the android. Actually the device consists of Fuel-Gauge which is the IC providing information  regarding monitoring of voltage, current, temperature. These values are then send to the concerned unit, for eg. charging current is send to the Charger IC etc.

The battery driver (not device) present in the device provides interface to the charger IC and Fuel Gauge IC (not in all chipset but in most of the Samsung phones) and has nodes which has these values and are used by the framework layer through sysfs. So the value which you read is the exact value read from the registers of FuelGauge IC presrent.

Hope this helps.