Fixed IWLAN handover issues

1. Fixed that fail cause is not included in
   PreciseDataConnectionState when handover fails.
2. Re-organized events registering/unregistering
   logic. When handover to IWLAN, some cellular
   specific events needed to be unregister, and
   vice versa.

Fix: 214670260
Test: atest DataNetworkTest
Change-Id: I74781e2765f340eb776f7e1f671ac7d17547bc64