Immediately tear down VCN network when entering Airplane Mode

This change immediately tears down the VCN network if all underlying
networks are lost, and the device is in airplane mode. This ensures
consistency where other components may not understand the lingering
behavior of the VCN network

Bug: 207014742
Test: atest FrameworksVcnTests
Original-Change: https://android-review.googlesource.com/1908637
Merged-In: Ib05f0c4255e847eed7d8623b35c5a7dfedd920d6
Change-Id: Ib05f0c4255e847eed7d8623b35c5a7dfedd920d6