Add a test for testing if [start|finish]Op will be called when
[start|stop]VpnProfile is called.
Also add a test to ensure that the startOp will not be called
again when seamless handover is happened.
Bug: 197135981
Test: atest FrameworksNetTests:VpnTest
Change-Id: I778ea4edf7c4a2d20b3b8e1877e7942feac5372a