Fixed that data network not entering disconnecting

When the network is being torn down gracefully, it should
enter disconnecting state and send the corresponding
PreciseDataConnectionState for IMS service to perform
IMS de-registration work.

Fix: 225971458
Test: Manual & atest DataNetworkControllerTest
Change-Id: I5c9678c63cfae0f72d4c70a3d656e6d581efd0e0