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.
Is there any benefit (yet?) to disable gecko DeprecatedAdbScanner and replace it with the runtime?
If yes, that would be great to drop a not here.