SDK / exoplayer

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