Minor fixes to LockdownVpnTracker.

1. Remove the legacy network type. This is only used for logging.
   - Replace the logcat logging with the display transport
   - Remove the EventLogTags logging, since it's likely not
     actually used by anyone.

2. Remove code that checks for NetworkInfo objects in state
   FAILED, since LockdownVpnTracker can never have received any
   such NetworkInfo from ConnectivityService since lollipop.

Bug: 173331190
Test: passes existing tests in ConnectivityServiceTest
Change-Id: I66ed71e51ba18b95862f3a0a5df2775eecea501e