Add cldr translation for values-es-rMX

This stub locale allows devices to choose Spanish (Mexico) in the setup
app and Settings. There must be at least one translation in the
framework for the internationalisation system to present that locale as
a selectable option.

Test: manual (flash a device with PRODUCT_LOCALES += es_MX and use it)
Bug: 172918378
Change-Id: I0e04846a29b697d7f7acfde7c6aaa684512afbab