Use exports [Event | Action].type instead of strings when subscribing and calling

[BelfordZ]

    [Nit] What do you think about extracting this to a constant in NetworkController? This feels like the sort of knowledge that NetworkController should keep and expose to other places rather than copy around. Pro: more knowledge in one place. Con: Not sure if we've done that before for other controllers, so it would be a new pattern. Just a thought.

Originally posted by @mcmire in #903 (comment)

[desi]

Closing.