Fix import of Keras

[sampathweb]

With the change in keras3 to use src and api folders, the from keras import * inside keras_cv/backend/keras.py doesn't import the src namespace.

To fix this -

1. Renamed the keras.py to keras2.py
2. Handle the import of keras in backend/__init__.py

This change has no impact to the users since its only required for accessing private files in Tests.