Add new layers_util target to break some dependencies on Keras.

- Many tests still depend on Keras so adding back some of the Keras
  dependencies to :framework_test_lib.
- Adding new keras_lib target that will consist of much of the code
  being split from core TF. In future CL, will remove keras_lib dep
  from tensorflow/python:no_contrib.

PiperOrigin-RevId: 226380802