Add an imported representation of init ops for 1.x-style SavedModels

Adds TrackableAssets to a property of the root object, and adds a TrackableResource which runs a function containing the original init op. This allows repeated re-export of 1.x SavedModels without losing information about assets or initialization.

Uniquifies shared_names rather than clearing them. 1.x SavedModels rely on shared_names sharing resources across functions (e.g. the init function vs. the function that uses a table) so clearing doesn't really work. This will leak cached kernels, but at least provides the right behavior.

PiperOrigin-RevId: 233983446