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