Persist version data on a per-volume basis.

Now that we support multiple adopted external storage devices, we
need to keep track of version data for each volume.  This means we
now correctly handle certificate upgrade edge cases, permission
regranting, and clearing of code caches on a per-volume basis.

Bug: 22298966
Change-Id: Ifb9940c197f6c058a3ecca728257f853ce0fd7f4