In cases, where the caller is looking for the locale to be used for JS Intl API,
we can now replace it with `undefined` which causes JS Intl API to use the default
locale which since bug 1346674 is resolved to the app locale.
This allows us to remove a lot of calls for the app locale.
The remaining ones are split between `AsBCP47` and `AsLangTag`.
Here, the `AsLangTag` is used, as described in the API docs, for cases where
the language string is used for localization purposes, such as language negotaition
matching to our language resources etc.
`AsBCP47` is used when the returned value is handed over to ICU API.
MozReview-Commit-ID: DzmFEUvMq3N
--HG--
extra : rebase_source : 513ed31d995864939aa893e73c81ffdf591a6617
3d91da8726