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. 
isProxying is a bit confusing because it sounds like checking if this user (i.e. the user running uproxy) is getting access, but here its used for giving access.  Can you get rid of this property and just use this.accessIds (which might be better named something this.numInstancesGettingAccess) to tell if the user had been proxying or not?