Decoupled ResourceVariable from RefVariable

* RefVariable._try_guard_against_uninitialized_dependencies and related
  methods are now functions used by both variables subclasses
* Removed implicit chaining of ResourceVariable ->Tensor convertor with
  that of RefVariable. Previously, ResourceVariable._dense_var_to_tensor
  would return NotImplemented if called with a different dtype. This
  effectively delegated the conversion to RefVariable._TensorConversionFunction
  which allowed compatible but not exactly matching dtypes. The change
  makes this behavior explicit in ResourceVariable._dense_var_to_tensor.

PiperOrigin-RevId: 227872550