Themes: Ensure themed fonts always have fallbacks

We need to ensure that if fonts are being used from a theme that
we include any families from the system that the theme did not
include.  We also need to add the Roboto fonts as fallback.

The system sans-serif fonts (Roboto) are added as fallback fonts.
This ensures we have a fallback for glyphs that are not in the
themed fonts.

Change-Id: Iebb634ef4d01fedd132858410a5d392e74800acf