Make StorageVolume#getOwner public

Make StorageVolume#getOwner accessible to apps so
that if we have an API that can return volumes
from multiple users (like b/209973853), then the
caller should also be able to figure out which
user each volume belongs to.

Bug: 213658045
Test: atest
AppCloningHostTest#testGetStorageVolumesIncludingSharedProfiles

Change-Id: I8d1ccc621a8808f6f072d147888869013192494b