[bug 977270] Don't infer platform if products don't match

If you're writing up feedback for Firefox for Android using Firefox, we
shouldn't infer the product from the user agent because it'd be wrong.
For example, you'd get something like Firefox for Android feedback with a
platform of Windows... wat?!

This implements the logic in https://bugzilla.mozilla.org/show_bug.cgi?id=977270#c4

That won't capture platform for Firefox Metro (which is now defunct anyhow) or Firefox for Android, but I don't really care since we don't do platform breakdowns for those products. It doesn't affect feedback for Firefox OS since that goes through the API.

r?