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.
If I'm not mistaken, I think you can simplify this to:
```
return adb.shell("id", stdout => {
  let uid = stdout.match(/uid=(\d+)/)[1];
  return uuid == "0";
});
```