Fix Ikev2 vpn cannot establish through startLegacyVpn()

Settings will also call startLegacyVpn() to bring up Ikev2 VPN,
so it's needed to add a vpn type check in startLegacyVpn().
Otherwise, UnsupportedOperationException will be thrown when
Settings tries to bring up Ikev2 VPN.

Bug: 191937302
Bug: 188749090
Bug: 189808606
Test: 1. make gts && gts-tradefed
      2. run gts --module GtsNetworkStackHostTestCases
Change-Id: I752c39bd1bf953a11bdf62e6f02f99f48fef23c4