Reorganize aconfig flags

This CL includes
- Put all config entries into the single aconfig file.
- Remove custom_locale_fallback and deprecate_fonts_xml because
  Gantry doesn't support converting READ_WRITE flag to READ_ONLY flag.
  To be a READ_ONLY flag, need to create new flag and start from
  scratch.

Bug: N/A
Test: N/A
Change-Id: Iad6229b2a620674223037ccaf41ef5fcab8f7dea