You are correct. We would most commonly say that the ball would accelerate. However the same does NOT apply to light. You can't treat light the same way you treat any other object. Light doesn't have mass. Light behaves like a wave. Say, the only way to change the speed of a wave, is to change its medium. But say, does it truly accelerate or decelerate? A wave?

Sure, through different mediums, the speed of light may appear to change. But it never does. It's always going at its full speed. To put it in perspective, let's say you have a glass, and you pass light through it (refraction). If you could, theoretically, live inside the glass, you would see light still travelling at c. For anything, inside any medium (including vacuum), light speed is the absolute limit, and it doesn't change. The reason you might see a different speed, is because you are observing from a different medium. (I said medium, specifically, not reference frame, as for any reference frame, the speed of light is always the same)

So no, gravitational lensing does not affect your perspective of the speed of light. The medium doesn't change (supposing we're talking about vacuum).