[tf.data] Fix bug where captured resources in shared iterators were invisible.

This change ensures that a shared iterator (which requires a private
FunctionLibraryRuntime that outlasts the calling op's runtime, because
it can outlive a single session) uses the same Device as a non-shared
iterator, and hence capturing resources from the creating graph will
work as intended.

Fixes #16481.

PiperOrigin-RevId: 184172498