We previously used Locale.fromLanguageTag and then toLanguageTag to automatically
filter out invalid tags and to replace deprecated tags. However, this only works
for API 21+.
This change does no longer rely on the platform methods and instead:
1. Keeps invalid tags as they are. This may help if developers rely on
non-spec-complaint language tags (for example, #6681).
2. Adds a list of deprcated tags and their modern replacements to our code
3. Normalizes some short codes that have been superseded by macrolanguage codes by
their macrolanguage equivalent.
Issue:#6681
PiperOrigin-RevId: 285384841
