So what makes this specific to Android phones? Developers can't (get permission to) access this information on other platforms?
It's available on iOS: https://developer.apple.com/library/ios/documentation/UIKit/...
The research paper says they're doing this ...
... by repeatedly reading the following two files:
/sys/class/power_supply/battery/voltage_now
/sys/class/power_supply/battery/current_now