Add a way to get the file directory used to store the new seed file.

This CL adds a getter to VariationsServiceClient classes for
retrieving the file directory that the new seed file will be stored
in. This is to support this request: https://chromium-review.googlesource.com/c/chromium/src/+/5874123/comment/ae79fac8_fc63a298/

* Expected behavior for platforms that support the new seed files
(desktop, Android Chrome, and iOS) is to return the User Data directory.
(Separately, Shell returns a User Data directory, too.)

* Expected behavior for the remaining VariationsServiceClient classes
(Android WV and headless) will be to return an empty file path as they
will not be using the new seed file and will continue using local state.

* Also re-orders some functions to match VariationsServiceClient's
order for best practices.

Bug: 365079796
Change-Id: I7dd025bff573ef77d88aaf44849803758b787ae4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5909666
Reviewed-by: danakj <danakj@chromium.org>
Reviewed-by: Caitlin Fischer <caitlinfischer@google.com>
Reviewed-by: Ginny Huang <ginnyhuang@chromium.org>
Commit-Queue: Cristian Lopez <cristiandlopez@google.com>
Reviewed-by: Luc Nguyen <lucnguyen@google.com>
Cr-Commit-Position: refs/heads/main@{#1365005}