Use public keras register throughout repo

[seanpmorgan]

Since register_keras_serializable was added, We should use that instead of our internal utility and remove that from the repo:

https://github.com/tensorflow/tensorflow/blob/master/tensorflow/python/keras/utils/generic_utils.py#L134

@karmel just want to confirm that our package name should be Addons

[karmel]

That SGTM-- matching the ops namespace makes sense. The scope will be within users' code, so doesn't need to be globally unique in this case.