[base] Replace SkFontMgr::RefDefault() with FreeTypeFontMgr

Android Renderengine doesn't draw any text, but hwui needs
to turn bytes into SkTypefaces and it currently uses Skia's
"Empty Custom" SkFontMgr, which is really just a wrapper around
FreeType. This makes that connection explicit instead of relying
on SkFontMgr::RefDefault(), which is going away soon.

Change-Id: Ida96538c76e069ec86e8dc078ef329cfaeac5d78
Bug: b/305780908