trekbuddy is an awesome offline program I have been using for years and works on my android. It has offline maps which you can make yourself too.
It s really simple process on trekbuddy to do what you are asking. You just put your A to B gpx file in the tracks folder. Start your GPS and get a lock. Then in trekbuddy you would scroll down the list of waypoints which will show up when you open the gpx file and simple pick the very last waypoint which in your case should be location B and then click navigate to/navigate along.
Its that simple.
Of course it will guide you to location B from C... but it wont be a turn by turn navigation etc like on Google navigation but it will give you a compass, show the map, allow zooming (depending on how you create your 'atlas').
For example... I did a walk one using trekbuddy and I walked very far and could not have retraced my route backto the car. So instead, I just stopped the gps logging and opened up the gpx saved file in trekbuddy, picked the first waypoint which in my situation was were I started from and where my car was parked... then I simple headed in that direction using the trekbuddy compass and occasionally looking back at the map. It was that simple and got me back to my 'A' starting point on a 'as the crow flies' route.
Hope that helps.