let forwardPort caller specify remote port; use correct API to track devices

@ochameau: Here are the fixes needed to get libadb.js working. With these fixes, I am able to connect to an Unagi running the latest nightly central build, install an app to it, and debug the app.

I did experience a number of issues while debugging, like the Inspector tab not working. But presumably those are unrelated to the core functionality being provided by this addon.

The other issue I see is a Firefox hang on shutdown after doing `make run` with the addon installed. It isn't necessary to open the App Manager nor even connect a device. I'm testing on Mac, haven't tried any others yet.  I don't think we're experiencing that with the Simulator 5 preview builds, although @bdahl or @bkase may know differently.