Currently, the network controller notifies consumers about events by
emitting them directly from the controller. In order to migrate the
controller to the core repo, where controllers use the BaseControllerV2
interface, events should be emitted via a messenger object.
This commit updates the network controller to use a messenger, and then
updates all of the controllers that listen for network events to use the
messenger as well.
The network controller module has been renamed from `network.js` to
`network-controller.js`. All of our newer controllers have "controller"
in the module names, so this aligns better with that convention. It
also brings the test module name into alignment (it's already called
"network-controller.test.js").
