Add Firefox uninstalls as a baseline/default outcome

[ncalexan]

Right now, an experiment could drive users to uninstall Firefox entirely -- surely a decisive vote of non-confidence -- and that would not register in our outcomes (beyond measures of user engagement and retention). This ticket tracks counting users that uninstall Firefox. I think this should be a baseline/default outcome, just like, e.g., search volume. (Note that uninstalls are a Windows-only concept; there is no uninstall on other platforms. And not all Windows users installed in the first place, so not all can uninstall.)

Technically, this is a little awkward but not impossible. Uninstall events send a special uninstall ping. That ping contains the (legacy) client ID of the profile that last ran that Firefox installation. I manually counted such uninstallations for a native notification experiment: see https://docs.google.com/document/d/1zuFv8LX0o-5lTjxQzrEfDHPJZVKNQfsDrjOJagGWa2k/edit# and my notebook at https://colab.research.google.com/drive/1-kXArtZMU0wrsieez60iqXZom19XPOOG.