Update TFLite concrete function conversion codes to have the associat…

[abattery]

…ed trackable object.

Providing trackable objects is now recommended for the users. This makes the concrete function conversion API be based on the new SavedModel importer, which will enable new TensorFlow Lite features including variable support, resources and variant tensor, and signature concept.

[abattery]

@hmoraldo could you take a look? Thanks :)

[hmoraldo]

Thank you for the contribution! This looks good for me.

Very likely we'll stop reviewing pull requests after this one, because unfortunately we won't have the bandwidth to check future changes in the code.