Use `Locale.US' for string comparisons in search

Use a fixed locale for comparisons to make sure they're the same across locales (eg. if the user is searching for a Turkish uppercase dotless `I` you probably want it to be converted to a lowercase dotted `i` for searching instead of a Turkish lowercase dotless `ı`).

See [`Be wary of the default locale`](https://developer.android.com/reference/java/util/Locale.html#default_locale) for a discussion of this (though it's more geared towards machine readable output).