Add an option to user dict to match more precise locales

This should not be used lightly, as it violates the general
contract of locale, and does kill some legitimate (albeit
alledgedly rare) use patterns.
Currently, the spell checker uses this because it uses a
negative logic: it should match broadly, and when in doubt,
match even more broadly, which is almost never the case of
something that uses the locale.
In other words: don't use this option unless you are
very, VERY sure that's what you want. Hint: it isn't

Bug: 5280929
Change-Id: Ib3cae319c692161d653630038c5bcde1f4340c05