Verify VPN can handle network loss

This commit also
- verifies that during network switch with MOBIKE
  enabled, the cleanup task is scheduled upon network loss and
  cancelled when new network is available.
- improves the retry tests to avoid waiting for actual timeout

Bug: 192077544
Test: atest VpnTest (new tests)
Change-Id: I5b47ba98116ac4523a36bc495e8788f29a9ecf20
1 file changed