A light might not help you. The police might think that it's a laser sight, and be even more likely to shoot you:

The officers claim that the Taser projected a red light, which they assumed was the laser sight of a handgun, and feared for their lives. At 7.18:43pm, Schiff and Sawyer began barraging Nieto with .40-calibre bullets.

http://www.theguardian.com/us-news/2016/mar/21/death-by-gent...

Flashlights are also commonly attached to firearms.

In fact, it would be hard to distinguish between someone holding a phone with a light on or someone holding a gun with a light attached.

I'm not defending police brutality, but these are often difficult situations.

Well, people have been shot with far less obvious things on their hands mistaken (or "mistaken") for guns.

Besides, what phone camera has a light on it that's visible?

None that I can think of in day/noon/afternoon time. Some have a focus assist light and/or flash at night time -- but taking out of one's pockets and pointing a small object at a US policeman at nighttime, and especially in an aggressive scenario as the one we're discussing, that I would advise even less.