Fixed dangling network agent not cleaned up

When a network agent becomes dangling due to
IWLAN handover failed, telephony failed to clean
it up because the state did not transist from
CONNECTED to DISCONNECTED. In this case we need
to explicitly call unregister.

Fix: 156443459
Test: Telephony sanity tests
Merged-In: I6fcf1895964f66ef1095f62475fb856b10ce4948
Change-Id: I6fcf1895964f66ef1095f62475fb856b10ce4948
(cherry picked from commit ec05e7f2)