Themes: Add system font families missing from theme

A theme's fonts may not include all the families defined in the
system's system_fonts.xml.  If a system font family is missing
from the theme's fonts, we add it.

This works for missing monospace fonts as well as other families,
such as serif.

Change-Id: Ib2a71fab582cfdf54f4d374d0c32cfebfaa1564f