[Refactor] Rename SigninManager.isSigninAllowed() to isSyncOptInAllowed()

SigninManager.isSigninAllowed() checks for ConsentLevel.SYNC instead
of ConsentLevel.SIGNIN. Thi cl renames the method name to better reflect
what it does.

There is no behavior change introduced by this cl.

Change-Id: I18c1904f4568a6b794b84bb4098399afc7a73c0d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3380524
Reviewed-by: Boris Sazonov <bsazonov@chromium.org>
Commit-Queue: Tanmoy Mollik <triploblastic@chromium.org>
Cr-Commit-Position: refs/heads/main@{#958070}