Fix NullPointerException in Ikev2VpnTest for watches

Watches do not have a VpnManager instance anymore, so the tests crashes
at the teardown step when trying to interact with a null VpnManager.

Bug: 368624224
Bug: 340928692
Test: Ikev2VpnTest
Change-Id: Iedf784625933e73bcb5fed9bfa36fbae1e11d398
1 file changed