Fix: VPN is off but key is displayed in the status bar

There seem to be a narrow race condition in which it's
possible to turn the VPN off where it won't null out
the config. It's possible to make this happen by trying
to migrate to a network that won't carry the VPN packets
(UDP 4500 or ESP, respectively) and turning off the VPN
while the IKE session is trying to establish.

Alternatively, this might be a visibility issue where
mConfig is nulled out by a thread, but the binder
thread reading it from #getVpnConfig returns the old
object, not yet nulled out. To fix this, make mConfig
guarded by "this", which it almost was already.

Bug: 289187571
Test: manual
      Turning off underlying networks, VPN stays "connected"
      in settings
      Adding a settings VPN, making sure the status is correct
      Try and fail to cause the race condition
(cherry picked from https://android-review.googlesource.com/q/commit:25181986301828f8be5101bb6b08a72dda99b0ad)
Merged-In: I1c19eb33f0c1d0f1ccca6ac17a709f8bc1ad9f94
Change-Id: I1c19eb33f0c1d0f1ccca6ac17a709f8bc1ad9f94