Ensure all VPN runners clean up state when exiting

This CL tweaks the cleanup flow to ensure that VPN runners exit
properly and clean up state. Previously, if a VPN exited before an
interface was created and the Interface Observer started watching the
virtual interface, some state (eg mConfig) might not get cleaned up.

Also as a result of this change, the LegacyVpn no longer implicitly
relies on the NetworkManagementEventObserver's watching for interface
removed to cleanup mConfig, mStatusIntent, mNetworkCapabilities, and the
VPN runner itself, but rather clears the state immediately.

Bug: 144246767
Test: FrameworksNetTests passing
Change-Id: Ide9daebca9a3fba025e7da5e3fe1d20d7bfdca02