Added network agent auto clean up logic

In some rare cases, the netwotk agent would become
dangling and make connectivity service thinkgs the
network is still alive, hence killing the network
brought up later. Added the agent self clean up logic
in network agent to auto clean up if onNetworkUnwanted
is called for 30 seconds, but without unregistering
from connectivity service.

Note this is a temporary solution for S QPR. This cannot
happen on the new architecture on T because there is no
such transferring network agent between data connections.

Fix: 221956304
Test: Sanity tests + atest DcNetworkAgentTest
Merged-In: Iee219a43d70aca80e5a87b5c22305fbc57768654
Change-Id: Iee219a43d70aca80e5a87b5c22305fbc57768654