[ios] Support send-tab-to-self promo when the are 0 device accounts

Behavior changes are behind flag.
Previously, no STTS sign-in promo was shown to users with 0 device-level
accounts, because the sign-in sheet didn't support adding new accounts.
kConsistencyNewAccountInterface adds such support, so show the promo
if the flag is enabled.
Also remove an unnecessary null check for ChromeAccountManagerService
(the service can't be null outside of incognito).

Bug: 1447012
Change-Id: I8ce7b25b181084881fc8f426390d89d517e4da0b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4625199
Commit-Queue: Victor Vianna <victorvianna@google.com>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1159925}