Deprecate fonts.xml and add hidden font_fallback.xml

Not to break the apps that reads fonts.xml and add new features to
the system font customization at the same time, this CL introduces
new hidden XML file font_fallback.xml which is currently a copy of
the fonts.xml.

The fonts.xml still exists but may not contain the latest system font
settings due to compatibility problems. Application should use the
public API for accessing system installed fonts which is available
from API29.

Bug: 281769620
Test: atest CtsGraphicsTestCases
Test: atest CtsTextTestCases
Change-Id: I556b5fffb0c78f0c6150b472fd240b9a546de93f
Merged-In: I556b5fffb0c78f0c6150b472fd240b9a546de93f