[sync] Avoid problematic kSync cases in integration tests

There are plans to disallow `ConsentLevel::kSync` in this particular
API, so this patch migrates callers to
`SyncSetupHarness::SignInPrimaryAccount()` that use ConsentLevel::kSync.

These cases were particularly problematic because they run into an
obscure test-only behavior on ChromeOS, where access token fetching
could end up being completely broken. As a workaround, this patch
special-cases the handling of the ConsentLevel::kSignin->kSync
transition in SyncSigninDelegateDesktop.

Change-Id: Ibc55e497617741b96d9e28130469078bc97bbabf
Bug: 41496149
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6590591
Reviewed-by: Alex Ilin <alexilin@chromium.org>
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1466469}