Choose a default hour cycle for a locale when no preferences are provided

This is a follow-up from https://github.com/unicode-org/icu4x/pull/587#discussion_r605070056

> [When no preference is provided the preference] should default to the locale default, which is region-based (h12 for US, h23 for GB, etc). This is in CLDR, but we need to load it as well as think about the best architecture for where that data loading code should reside.
