Migrate `enableSpannableBuildingUnification` to new feature flag system (#42716)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/42716

Migrates this to the new flag system just added.

Made a quick change to the Android template, to mark Kotlin accessors as `JvmStatic` to make calling from Java more idiomatic.

Next diff will wire to MC.

Changelog: [Internal]

Reviewed By: rubennorte, mdvacca

Differential Revision: D53198874

fbshipit-source-id: 6ab5b279d9ac59733c6e820c25be72383ce0e54a