This lays the groundwork for variation settings support.
Since we should regard different variations of a font as different fonts, we
need to create new typefaces. To reuse the same instance of MinikinFont, as
much as possible, FontFamily::createFamilyWithVariation now reuses an
existence instance, while incrementing the reference count.

Test: minikin_tests
Bug: 33062398
Change-Id: I08e9b74192f8af1d045f1276498fa4e60d73863e