StorageManager: Properly convert FAT volume identifiers.

FAT volume identifiers are randomly generated 32-bit identifiers
that are not UUIDs as per the spec. However, we need to coerce
them into UUIDs because several storage APIs are defined in terms
of UUIDs.

We need a follow up change to properly return UUIDs for public
volumes in order to complete developer support.

Test: atest StorageManagerTest
Bug: 166129035

Change-Id: Icaa4f3485af698ab7163031c540aa2b27d1e1f16