Lockdown VPN handles its own connection teardown.

Recent changes started watching for CONNECTIVITY_ACTION broadcasts
to handle the case where a network is disconnected without the
interface going down.

However, when lockdown VPN is enabled, the broadcast contents are
augmented, and all connections appear disconnected until the VPN
comes online.  This caused a reset feedback loop to occur.

Since LockdownVpnTracker already handles networks being disconnected
separately from interfaces going down, this change disables handling
the broadcast when lockdown is enabled.

Bug: 8755148
Change-Id: I70a348aa97a4b22eaaf23aa5ed344de3e9a9ab0b