Use separate XML for update config in system

This CL is a refactoring to address following things:
- Separate XML definition of fonts.xml and update.
  Since there are lots of concept mismatches between system font config
  and update config, e.g. file paths, family variants, etc.
  It is good to manage separtely.

- Stop keeping udpated config in memory in FontManagerService.
  As a side effect of above change, FontManagerService no longer keeps
  FontConfig.FontFamily mappings. This will be created from XML file
  when necessary. This is better in FontManagerService since this map
  is only necessary the boot time. After the system is booted, this
  map is only necessary when the update request is called from client
  which happens quite rare.

Bug: 183752879
Test: atest UpdatableFontDirTest
Test: atest FontManagerTest
Test: atest UpdatableSystemFontTest
Test: atest FontFamilyUpdateRequestTest
Test: atest PersistentSystemFontConfigTest

Change-Id: If2c12a8029f859efc52721b07cfeab04be2deb78