Negative currentmA and Shuntvoltage
The "sign" of the INA219 shows the direction of flow of the current. Current isn't either "positive" or "negative", it has a direction. The sign is just used to represent that direction.

It's like a car. It has a speed. That speed could be in the forward direction, or the reverse direction.  It could be going 5 miles per hour forwards, or 5 miles per hour backwards. Either way, it's 5 miles per hour. However the backwards direction could be represented as "minus 5 miles per hour". It doesn't mean that you're doing a negative speed (there is no such thing), but that your speed is in a "negative direction".

This "speed" and "direction" are combined together to create "velocity", which is "speed relative to some arbitrarily chosen 'normal' direction and speed".

At this point the "velocity" can be represented as a 2D vector, with forward velocity in the + direction, and reverse velocity in the - direction. 

But that's all a bit of a digression. All it means is that the "-" is meaningless if you don't care about the direction of current flow. If all you care about is the quantity of current you can discard the "-" completely.  The simplest way is:

current = abs(current);