Test VpnProfile is not modified in startLegacyVpn

The profile is modified within startLegacyVpnPrivileged(), e.g.
ipsecCaCert, ipsecUserCert. This causes subsequent calls from
callers such as LockdownVpnTracker to fail.

Add a new test to ensure the VpnProfile is not modified in
startLegacyVpn.
Add testClone in VpnProfileTest.

Bug: 190572822
Bug: 309591931
Test: atest FrameworksNetTests, test before and after fix.
Test: Manual test
Change-Id: Id090e0dd3b9e74a7461ae90e68c41e802b458744