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A bunch of passwords/encryption-related helpers existed in
passwords_helper/encryption_helper but also had almost-identical copies
in single_client_passwords_sync_test.

This CL moves all the helpers into passwords/encryption_helper and
de-dupes the almost-copies -> less duplication, and they can now be
used in additional tests (like PasswordManagerSyncTest).

Bug: 1058339
Change-Id: I5bae176053ca6c2a063d09fa3a80efb116df4003
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2101000
Commit-Queue: Marc Treib <treib@chromium.org>
Reviewed-by: Maksim Moskvitin <mmoskvitin@google.com>
Cr-Commit-Position: refs/heads/master@{#750579}