Fixed IWLAN handover issues

1. Fixed a race condition issue that when connectivity
   service release network while handover is ongoing,
   TelephonyNetworkFactory incorrect added the network
   request back to the network request list. This caused
   double handover request afterwards.
2. Fixed that DcTracker cannot handle the 2nd handover request
   correctly. The 2nd request was failed immediately caused
   upper layer think the handover has already failed.

Fix: 186588930
Test: TelephonyNetworkFactoryTest, DcTrackerTest
Change-Id: I96d5f7f7f8f39abcb8cdae98af0c92281a057163