[Extensions UNO] Rename and migrate extensions UNO feature flag

Migrate the feature flag from syncer:: to signin switches since this
will control more than just sync (including allowing users to perform
an explicit signin via the extensions signin bubble.)

There is no behavior change in this CL as this is just a simple move
and rename.

Bug: 397720943
Change-Id: I7a9dbcb225d9d49b984df8e739757e2e1112d552
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6284365
Auto-Submit: Kelvin Jiang <kelvinjiang@chromium.org>
Commit-Queue: Mihai Sardarescu <msarda@chromium.org>
Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Reviewed-by: Devlin Cronin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1423098}