Create fonts.xml module for adding fonts.xml to system/etc

Previously, we used PRODUCT_COPY_FILES. However, that does not allow
overriding the file to use a different one on a per-device basis.
This change migrates the logic to using a BUILD_PREBUILT module
called fonts.xml.
To add additional fonts, the add_additional_fonts.py script has been
added. If ADDITIONAL_FONTS_FILE is defined, add_additional_fonts.py
is run and generates a new version of fonts.xml that contains the
content of ADDITIONAL_FONTS_FILE added where
"<!-- fallback fonts -->" is added.

Bug: 36855735
Bug: 37997769
Test: manual. built multiple devices and verified that the correct
versions of fonts.xml were showing.

Change-Id: I8308f0cb3068274bca5ebd25a38cc023babbd69d