Switch to new WebIDE scanner / runtime API (bug 1069552)

Makes use of the new scanner / runtime API in WebIDE.

After this change, the add-on will still work with pre-API Firefox.

I've also added a Firefox for Android support, as it was helpful to build it to try out the runtime API.
Sorry, I should have explained this more...

It's actually still useful to have Devices.jsm as a registry of low-level / ADB devices. The WebIDE API changes mean that WebIDE no longer depends on it directly, so WebIDE is more flexible.

If the only use for these low-level ADB devices was to find Firefox OS runtimes, then I would agree, we could remove Devices.jsm. 

However, other add-ons that know about other runtimes on ADB devices can actually use the Device instance to find even more runtimes. In fact, that's exactly what I do in Fever Dream to find Chrome on Android in campd/fxdt-adapters#82. So, the Devices.jsm registry still has value.