Avoid user autocreation race condition caused by repeatable read

Avoid user autocreation race condition caused by repeatable read

AuthManager tries to check whether the user already exists if
User::addToDatabase fails in autocreation, but since the same DB row
was already checked a few lines earlier and this method is typically
wrapped in an implicit transaction, it will just re-read the same
snapshot and not do anything useful. addToDatabase already has
a check for that so let's rely on that instead.

Bug: T145131
Change-Id: I94a5e8b851dcf994f5f9e773edf4e9153a4a3535