turn off autoGainControl in webcams

[jywarren]

https://w3c.github.io/mediacapture-main/getusermedia.html#media-track-constraints shows that at least the spec contains this!

It'd probably involve passing constraints like this:

{
    video: {
      autoGainControl: false
    }
  }

[sidntrivedi012]

@jywarren Would like to work on this. Can you guide me about the file where this change should be made? I tried to find but couldn't.