[skyvault] Change migration policy to enum

Adds LocalFilesMigrationDestination policy instead of the boolean LocalFilesMigrationEnabled, and updates the corresponding pref and usage in LocalFilesMigrationManager.

Although a boolean policy, as originally planned (added in https://crrev.com/c/5576818), is simpler to implement, using an enum instead makes it easier to support future modifications e.g. adding an option to delete the local data instead of moving to the cloud.

R=ayaelattar@chromium.org, poromov@chromium.org

Fixed: b/343400162
Change-Id: I67be1a2e90099abc0195f57763b79d7ccedc5815
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5612350
Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
Reviewed-by: Sergey Poromov <poromov@chromium.org>
Auto-Submit: Aida Zolic <aidazolic@chromium.org>
Commit-Queue: Aida Zolic <aidazolic@chromium.org>
Reviewed-by: Aya Elsayed <ayaelattar@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1316409}