Moved `Variable.constraint` to Keras.

* The signatures of `VariableV1` and `variable_scope` were unchanged.
* `backend.variable` and `Layer.add_weight` now manually set the
  `constraint` attribute instead of passing it to `Variable`.
* `ResourceVariable` and `Variable` no longer accepts `constraint`
  in the constructor, nor expose it as a @property. This is a
  non-backward compatible API change but it only affects
  `ResourceVariable` users.

PiperOrigin-RevId: 225589244