Are you sure that it is impossible to detect it, even if the app is 'calibrated' to a certain object, e.g. an apple? I think if you limit the search space it could be possible!?
Spectroscopy is predominantly done with UV (200-280 nm most common) and IR, which are regions where photonic interaction is dominated by electronic and vibration/rotational transitions, respectively.
Visible light absorption is typically caused by highly conjugated bonds and metal-coordination complexes. In terms of day-to-day, this is almost exclusively dyes (synthetic and natural). Dyes also tend to be really potent absorbers - you only need minuscule amounts of them to create very vivid colors. So a purely visible-light-based app would at best be able to give you a handle of what sort of dyes are in something. It won't tell you if it has pesticides (let alone traces!) or HFCS or nutrients or what-have-you.
tl;dr - no, it's not remotely possible to even detect pesticides with visible light.
Most chemicals have characteristic spectra in the infrared, so you'd need sensors going to much larger wavelengths to have significant difference - even then it would be hard to detect against the variation in signal from fruit.
There's also no way you're measuring pesticide residue with that - I doubt that would even be possible with a high-end visible hyperspectral camera. Maybe with a raman spectrometer.
I've designed a couple versions of cell phone camera-based spectrometers and spectral imagers, so I'm relatively familiar with the design principles.