Remove LockdownVpnTracker from testLegacyLockdownVpn

The test coverage of LockdownVpnTracker has been moved to a
separate unit test file. testLegacyLockdownVpn now calls the Vpn
methods directly instead of creating a new LockdownVpnTracker.
Note this removes calls to expectStopVpnRunnerPrivileged since
stopVpnRunnerPrivileged is now directly called in the test so it
is already guaranteed to be called.

The expected calls/behavior of LockdownVpnTracker can be seen in
LockdownVpnTrackerTest.

Bug: 230548427
Test: atest FrameworksNetTests
Change-Id: Id9d26435bf62ffef954f6c7fa0558ce99540de1f