Include "invisible" volumes in new storage API.

The purpose of the new StorageVolume API is to grant access to
volumes that aren't typically "visible" to a developer, so include
them in the returned results.

Also return the real mounted state instead of augmenting based on
the caller's storage permissions.  Clean up API naming slightly and
return as List.

Bug: 27615770
Change-Id: Ida921a4b91e5af81e418e76f672d9108f45a9781