Bug 1787790: Normalize system fonts with RFP on. r=emilio

System fonts can leak any user customization of system fonts, or user's
locale (e.g., en-US and ja Windows have different system fonts).
Also, Linux distributions/desktop environments set default fonts in
different ways.

Customization can be detected either with font metrics, the font allowed
list is not enabled or the font is included in it, or with
getComputedStyle, that leaks the name of the font that Firefox tries to
apply.

This patch try to prevent these leaks by using a generic "sans-serif"
for all system fonts, except on macOS, where it uses "-apple-system",
and on Android, where these fonts always use Roboto.

Differential Revision: https://phabricator.services.mozilla.com/D163576