Add StorageVolume#getStorageUuid

Unlike getUuid that returns fsUuid, this returns the 'converted'
fsUuid. This means that for the default internal storage:
- fsUuid = null
- uuid = UUID_DEFAUlT

Test: atest StorageManagerTest#testGetPrimaryVolume
Bug: 170481432
Change-Id: I4c590e5536326e5c2a0b62520f9853de680027ae