Update testLegacyLockdownVpn to mock VPN.

Override more VPN methods to mock the VPN interaction of
testLegacyLockdownVpn instead of relying on the Vpn class.
This includes:
    1. Overriding startLegacyVpnPrivileged() and avoid creating
       a VpnRunner.
    2. Removing expectStartLegacyVpnRunner() since it is not
       used when startLegacyVpnPrivileged() is overridden.

Bug: 230548427
Test: atest FrameworksNetTests
Change-Id: Id55d8d6cd03b84bca815cd331eb0f7d584eaed5f