Fix testLegacyLockdownVpn flaky

When WiFi disconnects, the VPN disconnects immediately. The
broadcast can therefore be sent before the broadcast receiver is
registered, which causes the receiver to not see the broadcast.

The puzzling part is that CONNECTIVITY_ACTION is a sticky
broadcast, so one would expect the broadcast to still be
received, even if the registration is done after the broadcast
is sent. The reason this doesn't happen is that the context used
by the test is a BroadcastInterceptingContext, which does not
treat sticky broadcasts as sticky.

Bug: 184115648
Test: atest --iterations 1000 'ConnectivityServiceTest#testLegacyLockdownVpn'
Change-Id: Ib44c92839d25951cc7d2db0f923e1b104690e1e0