Fixed dangling network agent issue

Fixed another corner case that if target dc
takes long time to handover but failed, and
source dc went away during this time, the network
agent's state might not be set to DISCONNECTED
properly. This can cause dangling agent at the
connectivity service layer.

Also improved the debug messages.

Test: Telephony sanity tests
Bug: 139493331
Merged-In: Ie20db736992ae1320b391c5f27a1c2bf120529f0
Change-Id: Ie20db736992ae1320b391c5f27a1c2bf120529f0
(cherry picked from commit c337a98b)