[TF/XLA] Optimize `Encapsulator::GetFunctionNameAttr()`.

The previous version was hitting a very slow path in `GetNodeAttr()`, which is expensive when the named attr is not found. This change inlines the logic of finding the two relevant attrs inside `GetFunctionNameAttr()` and avoids constructing a status object with a serialized `NodeDef` when the attr can't be found.

PiperOrigin-RevId: 215298411