Fix KerasLayer compatibility with Keras 3.x/TF 2.16+

[athulya-anil]

Fixes #101201

• Replaced deprecated _add_existing_weight implementation that previously relied on unsupported arguments (experimental_autocast, getter) removed in Keras 3.x.
• Introduced custom weight lists (_hub_trainable_weights, _hub_non_trainable_weights) instead of using the deprecated _track_variable approach.
• Overrode trainable_weights and non_trainable_weights properties to properly expose hub weights to the Keras layer system.
• Resolves the ValueError: Only instances of keras.Layer can be added to a Sequential model and related integration failures when using hub.KerasLayer in Sequential/Functional models.

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