2-letter codes (ISO 639-1) are the standard Android normalization and thus we
should prefer them to 3-letter codes (although both are technically allowed
according the BCP47).
This helps in two ways:
1. It simplifies app interaction with our normalized language codes as the
Locale class makes it easy to convert a 2-letter to a 3-letter code but
not the other way round.
2. It better normalizes codes on API<21 where we previously had issues with
language+country codes (see tests).
3. It allows us to normalize both ISO 639-2/T and ISO 639-2/B codes to the same
language.
PiperOrigin-RevId: 258729728
