Fix AlwaysOnVpnTest flakiness

Wait for loss of connectivity instead of asserting
immediately, to workaround VPN setup asynchronicity.

Bug: 180520715
Test: atest --it 30  MixedProfileOwnerTest#testAlwaysOnVpn
Change-Id: Ie9f7ac1625a988db3bb4d2834d4c5ccea41859f5
1 file changed