Fixed duplicate interface detector

When clearing the dangling network agent, the interface
name is not availalbe because the handover destination
data connection was not succesfully setup, so there is no
valid interface name.

Changed the detection logic to use netId/iface-name map
to make sure all live agents do not have duplicate interface
name.

Fix: 161074519
Test: Manual
Change-Id: I7b0f20bb499cd50978206265707a806521f08437