Update extension icon when proxying to others

Ran grunt test.

Icon updates correctly when you start proxying, stop proxying, start providing (giving) but not when you stop providing. Should work once rtcToNet (in RemoteInstance) knows to close when corresponding socksToRtc is closed. 
It might be easiest to read if we just get rid of _setProxying and update the icon directly from the calling functions (there are only 2 of them, both in this same file).  The name _setProxying has been too vague