Add setting to attenuate output while speaking

On some handsets the audio output causes crazy feedback when you speak. This setting lets you lower the volume automatically when you speak to avoid this problem.

I'm still not sure if attenuating or just flat out muting (a checkbox instead of a slider) would be better. On my Gnex handset even at 90% attenuation other people can hear themselves talking while my mic is on. It's not nearly as bad as not lowering the volume at all, though.