This function was sync and was returning the overlay to let other parts of the code to update the content of the overlay (in some cases a progress).

If you take a look to line:

https://github.com/KevinGrandon/gaia/blob/4f481d7c8c5337a230bd2b03bac9bd4270d44363/apps/communications/contacts/js/views/settings.js#L518

You'll see in the import functionality that we expect to use the overlay, so we will need to transform the places where we use it to asynchronous calls.