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This CL refactors IsAllowed for borealis into the standard GuestOs
approach (i.e. having a pref-keyed singleton). This necessitates minor
refactorings of the installer and its tests.

Relatedly, this calls for the creation of Borealis prefs, which were
also added in the same fashion as the other GuestOs implementations.

Bug: b/168763038, b/162562622
Change-Id: Ie153c2a860a174feaa3156801a8415ca99eefcad
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2415755Reviewed-by: Nancy Wang <nancylingwang@chromium.org>
Reviewed-by: Dominic Battré <battre@chromium.org>
Reviewed-by: Daniel Ng <danielng@google.com>
Commit-Queue: Nic Hollingum <hollingum@google.com>
Cr-Commit-Position: refs/heads/master@{#813976}