It appears to be well within the normal sensitivity variation range for a cheap CCD (which is what cellphones use - a high-quality CCD [the light-sensitive part of the camera] sells for more than most cellphones). I wouldn't even bother contacting the manufacturer about it. (It's like people complaining that their cheap LCD TVs, the ones with 2-line LED lighting, are dimmer at the center of the screen. The real world and perfection aren't the same thing, they're not even distantly related.)
If you want a very even picture of a pretty solid-color scene (or object or whatever), expect to pay at least $500 for a camera that will come close. One that will give you a 16'X20' reproduction of a gray scale card that can be used as a gray scale card? Maybe a camera in the $2,000-$3,000 range, but even those will probably show some variation in detail at that magnification. (There's no such thing as a perfect CCD for any amount of money. The difference in a manufacturer's $500 camera and the same manufacturer's seemingly almost the same, but $2,000, camera is often nothing more than the CCD quality [they hand-picked the expensive one after extensive testing] and a few little niceties in the menu.) If you take snapshots, the camera in your phone is good enough or, if you need a bit more quality, a 16MP $75 point-and-shoot camera is probably fine. If the job is going to depend on absolute uniformity across the entire field with a very large blow-up, that one job will pay for a camera that will do the job (or buy you a small truck). Most people who depend on taking pictures as part of earning their livings (say real estate agents - and boy, could some of them learn a lot by taking even the first week of a 6 month "Photography 101" class - use phone cameras with non-flat distribution, and no one can tell the difference.
(How often are you going to be taking pictures of different quality paper? If you're the chief photographer of Hiromi Paper, have them buy you a Hasselblad and the finest grained film and paper made [and build a temperature and humidity controlled photo lab, because they affect picture quality too], and you'll stand a chance. We mere mortals don't worry about a 3% difference in density in one little spot in a field - that takes expanding the browser to the maximum size to really see.)
The real question is how good your phone audio is when you're out in the boonies and the nearest tower is so far away that you can't remember how long ago you drove past it. My camera doesn't pick up cell towers very well, and I don't worry about the density ratio of the CCD in my phone. I also don't turn screws with one of the claws on my hammer. Each tool has a specific purpose, and two of the purposes of the cameras in cellphones are to sell the phone to the public, and to allow you to take some pretty darned good-quality snapshots.
Someone once took a photo - with a Kodak Brownie, which is the equivalent of a $10 cellphone - of a snowy scene, with a black wrought iron fence in the foreground and a black car in the distance - with its red tail lights showing. Stark contrast between monochrome and color. And with a camera you gave as a really great birthday present to a 7 year old. The most famous photograph is Adams' picture of the moon over the desert. Just the usual moon that human beings have been looking at since there were human beings, just a normal desert with a few hills. Oh, in black and white. It's the Mona Lisa pf photographs.
Photography is the picture, not the technical details of the pixels that make it. There are people who could use camera with a lot worse CCD than your phone has, and win large prizes with the pictures they take. But your phone's camera is WELL within the limits of a pretty decent quality digital camera. If they replace it, or send you a new phone, I'd say your chances of getting a worse CCD are about 70%.
