[s13n] Move account_consistency_method.cc/h files to 'shared' target

This CL moves account_consistency_method.cc/h from the signin
component's 'internals' GN target to its 'shared' GN target as
it has only an enum.

Bug: 925345
Change-Id: I10873fba052a879d53f373b7b905c0aa4d472bc2
Reviewed-on: https://chromium-review.googlesource.com/c/1442039
Reviewed-by: Colin Blundell <blundell@chromium.org>
Reviewed-by: David Roger <droger@chromium.org>
Commit-Queue: Julie Jeongeun Kim <jkim@igalia.com>
Cr-Commit-Position: refs/heads/master@{#626948}