Navi Onboarding: add capability to return country-specific

This CL makes EmailHandler use the new country_codes component to determine
what data to send back based on country codes. This CL adds the mechanism,
and there will be follow-up CLs (marked with TODOs) to add the data for more
countries.

Change-Id: I2fc39ae5bd086a1ee8b2871c166a0f4d46e531b8
Reviewed-on: https://chromium-review.googlesource.com/c/1343192
Commit-Queue: Scott Chen <scottchen@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Hector Carmona <hcarmona@chromium.org>
Reviewed-by: Tommy Li <tommycli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611962}