Quick fix: throw non-file URLs into an Intent. Fix for 2189

From the Android docs [how to launch a browser](https://developer.android.com/reference/android/webkit/WebView.html), see Basic usage.
With an extra step added to check if the `url` actually looks like an URL

Should fix [issue 2189](https://code.google.com/p/ankidroid/issues/detail?id=2189)