Don't crash when touch actions sent to webviews

When the touch actions or multi touch actions are called outside of a native context, return `not yet implemented` error. Resolves #3518.
Is there anyway to detect this before we get to the device-specific code?

Eg, in controller.js, we'd have something like this:
```js
exports.performTouch = function (req, res) {
  // touch actions do not work in webview
  if (req.device.curContext !== null && this.curContext !== 'NATIVE_APP') {
    return cb(new NotImplementedError(), null);
  }

  // first, assume that we are getting and array of gestures
  var gestures = req.body;

  // ... etc, as before

```