Enable this user to run Crostini.
If the policy is set to false, Crostini is not enabled for the user.
If set to true or left unset, Crostini is enabled for the user as long as other settings also allow it.
All three policies, VirtualMachinesAllowed, CrostiniAllowed, and DeviceUnaffiliatedCrostiniAllowed need to be true when they apply for Crostini to be allowed to run.
When this policy is changed to false, it applies to starting new Crostini containers but does not shut down containers which are already running.