Fix dangling NetworkAgent

This change fixes the potential for a networkAgent to be left dangling,
due to a situation where a VcnGatewayConnection shuts down, but fails to
unregister it's NetworkAgent.

The root cause was that the NetworkAgent was not unregistered when
moving to the DisconnectedState from the RetryTimeoutState, and the new
state assumed that there was no NetworkAgent, and thus failed to close
it when disconnecting.

Thus, this change ensures that the NetworkAgent is closed before moving
to the DisconnectedState. Additionally, it adds safety-checks to
onQuitting(), ensuring that if all else fails, these fields are cleaned
up.

Lastly, this change adds the specific gateway reference to facilitate
future debugging of issues such as this where there is potential for
duplicate networks, or gateway connections.

Bug: 191707296
Test: atest FrameworksVcnTests
Change-Id: I84cd43a0c136662f5c2d229650f1f5f889e6f144