[Unity][Android] Implement bottom button bar in SyncAndServicesPreferences

This CL implements bottom bar with "Cancel" and "Confirm" buttons in
SyncAndServicesPreferences. This bottom bar is only shown when the user
opens SyncAndServicesPreferences screen from the sign-in screen.

Bug: 914056
Change-Id: I3f5f2002ab7bcd566f3305fd2344a79069bb82fb
Reviewed-on: https://chromium-review.googlesource.com/c/1435276
Reviewed-by: Theresa <twellington@chromium.org>
Commit-Queue: Boris Sazonov <bsazonov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#630098}
5 files changed