Make default resources a better match for en-US requests

When locale fallback landed, resources which specified an 'English'
locale started to be considered a better match for en-US, even though
traditionally, apps tend to ship US English resources under their
default locale.

This fixes that, and makes en-US requests match default locales.

Bug: 26756573
Bug: 26789680
Bug: 26803868
Change-Id: I460c276bfc6ddba0439dcdf87497a0aece0fa05d