Add 'export' method to TensorFlow Estimators and migrate callers.

The export.export_estimator function accesses protected methods on Estimators,
which is not only bad practice, but also makes maintenance/refactoring
difficult. This CL simply adds the 'export' method on Estimators and migrates
callers. A later CL will migrate the logic from export_estimator function to
the export method.
Change: 130651096