Use `_analogRead()` rather than `analogRead()` in `getBatteryVolts()`

[michaelherger]

https://github.com/pangodream/18650CL/blob/e1be2aa402bd19473aac2a843c9aeb4f274418c7/src/Pangodream_18650_CL.cpp#L152

You should use your own, internal _analogRead() function to respect the number of read iterations.