Add startOp and finishOp to track the behavior of VPN app

Record VPN appOps data is helpful in future analysis to identity
more PHA(potentially harmful application) VPN.

Bug: 197135981
Test: 1. atest FrameworksNetTests
      2. Manual test by using legacy VPN, VPN app which uses
         VpnService, VPN app which uses VpnManager
Change-Id: Id2d9ab9f76940fca3fd4263c707ea9198494e0d3