Unbreak extraInfo values returned to apps.

These were broken by aosp/1553463, which made filterNetworkInfo
unconditionally call setDetailedState with a reason of "" and an
extraInfo of null.

Fix both synchronous getter APIs (e.g., getNetworkInfo) and
CONNECTIVITY_ACTION broadcasts by calling a new
filterForLegacyLockdown method that behaves similarly to how the
now-deleted LockdownVpnTracker#augmentNetworkInfo used to behave.

While I'm at it, move back to private a method that was public
only because LockdownVpnTracker used it.

Fix: 181855958
Test: new unit test coverage
Change-Id: I2c7b88fcec9dd36b45cb51db8d19b3ee8bad44a6