Fixed dangling network agent issue

In some IWLAN handover failure cases, the associated
network agent might became dangling and causes out-of-sync
between connectivity service and telephony. Connectivity
service incorrectly thought the data connection was already
there so reaped the newly created one.

Bug: 138287040
Test: Manual
Change-Id: Idb1aa53d31f7b730946485b17fa088b97f3f9300