Read font info from system config files

This change adds new files in /system/etc: system_fonts.xml and
fallback_fonts.xml. The change also allows a third file to be added
by vendors in /vendor/etc/fallback_fonts.xml. These files, loaded at
boot time, mimic the logic that used to be in code, but which is
now configurable through editing these files, making the system more
flexible and easy to work with for us and vendors as future fonts
are added to the system.

Change-Id: I209f1ef08a5279b376b89a880241906387efd576