Expose fonts.xml via public API with a service

This change creates a new FontManagerService, in charge of providing
font management data. It exposes a public API to retrieve the
information in fonts.xml without accessing it directly. To do this,
it also refactors FontListParser's internal classes into a new public
FontConfig class holding all the font data.

getSystemFonts() returns all the available information in fonts.xml
as well as file descriptors for all the fonts. This allows us to
share the memory consumed by these files between all clients.

Bug: 34190490
Test: See attached CTS change in topic
Change-Id: I0e922f8bcc9a197a1988d04071eb485328d66fb7