FIX: set dom.w3c_touch_events.enabled=1 (fix NotSupportedError on desktop-helper touch-events.js)

b2g-desktops doesn't enable dom.w3c_touch_events by default, and if the preference is not defined, "document.createEvent('touchevent')" raise a NotSupportedError:

```
console.error: fxos_1_2_simulator:
JavaScript error: chrome://desktop-helper.js/content/touch-events.js, line 167: 
NotSupportedError: Operation is not supported
```

Using "dom.w3c_touch_events.enabled=1" fix the exception and "chrome://desktop-helper.js/content/touch-events.js" emulation helper works again (e.g. the FirefoxOS statusbar can be dragged again).