AthenaCommon: fix for private/public tool handling

Two fixes relate to private/public tools:
1) `ConfigurableService`: do not copy private tools when they are being
added to a service (issue gaudi/Gaudi#208).
2) Backport gaudi/Gaudi!1251 to correctly identify nested tools (e.g.
`ToolSvc.PublicParent.PrivateChild`) as private.