Fix network information type exception on fxos 2

`navigator.connection` got removed from fxos 2, causing the plugin to crash when loading. Fixed by returning  `UNKNOWN` when `navigator.connection` is not defined.

This is a quick fix to get this into next release ASAP, I'll now investigate if there's any alternative. There was nothing useful on [the mdn page](https://developer.mozilla.org/en-US/docs/Web/API/Network_Information_API).