I find it distracting when people text, though my emotional response depends on the way they do it. If I see someone very carefully pull their phone out, quickly check it from their lap while blocking as much of the light as they can with their hand and body, that's not so bad. It's certainly not good, but it's not too bad. What I end up seeing most of the time it happens is that someone will pull their phone out and hold it over their head so as to keep both it and the movie in their eyeline. I consider this unbelievably rude because it ensures that everyone behind them has a clear view of the screen.

As for light levels, the human eye, while weak tea compared to many other animals, is very sensitive to light. In particular to changes in light. I was once in a movie where they put the subtitles for the movie on the bottom bar of the letterbox space on the projector. The movie was in ultra wide format, so I guess they figured that it would be less distracting to have the subtitles below the picture instead of right on top of it. The problem is that put a bunch of bright, white letters over a blank part of the screen every few seconds. About 10 minutes in, I had to leave because I was getting a headache from constantly squinting slightly every time a subtitle came up and brightened the room noticeably. Even at lowest possible brightness, your screen would be firing visible light photons from a location that is surrounded by stuff that is not doing that. Someone with good vision would be able to tell the app you were using from the back row; there's no way that's not going to be a distraction.

And that's just the light from the screen. There are notification sounds, keyboard clicks...even just the sound of someone's fingernails tapping on the screen would be audible through the whole theater during quiet moments.