[skyvault] Allow OneDrive placeholder as default

Allows using the placeholder OneDrive as the default root, if it's set
so by policy but the real volume isn't available. This also addresses
the issue of removing the OneDrive volume while it's default.

R=lucmult@chromium.org

Fixed: b/340170015
Change-Id: I1e0c466e91104640a62c2fe0ecc0175e4e75625b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5543925
Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Commit-Queue: Aida Zolic <aidazolic@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1303696}