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. 
You can add a TODO here.  I think the problem is that some of our code (like chrome_connector.ts) thinks it just has a UIAPI object, and doesn't realize it really has a UserInterface object.  I've tried cleaning this up before too and always got stuck.  My understanding of the problem is that:
* in the core, ui should be a UIAPI object
* in the user interface code (e.g. polymer, proxy config, etc), ui should be a UserInterface object
* but at some places (possibly in firefox, also in Chrome if we eventually combine the app+extension), "ui" in the core and polymer/proxy/browser-action code will all refer to the exact same object, which can't have 2 types...