Treat Latin American locales specially

Treat Latin American locales specially

Due to legacy reasons, Android translations of European Spanish were
kept under 'es', while Latin American Spanish translations were kept
under 'es-US'. The combination of this, and the new locale
preference rules in Nougat, resulted in 'es' winning over 'es-US' for
all Latin American locales, since 'es' was a direct ancestor, while
'es-US' was just a fallback.

The changes in Nougat had assumed that app developers would put Latin
American Spanish translations under 'es-419', but that could create a
backward-compatibility problem under older Android versions that did
not support three-digit region codes properly.

This CL keeps the Nougat logic and its locale parent tree, but
special-cases es-US and es-MX to be treated as equivalents of es-419
in cases where they are present and es-419 is not.

Bug: 31545805
Bug: 34126460
Test: unit tests are included
Change-Id: Iab26f41294587ee044685a5a6560520c7cbb06f7