Refactor Operation.__init__ to create some state after creating _c_op

This change moves around the Operation.__init__ logic to create the
TF_Operation before initializing _outputs and before adding the op to
the control flow context (if any). This is in preparation for creating
Operation objects around TF_Operations indirectly created by the C API
(e.g. ops created by TF_ImportGraphDef).

This also disables running HessianTest with the C API enabled, since
it's broken for now (but will be fixed soon).

PiperOrigin-RevId: 175910443