Allow VPN apps to ask for running the validation checks

Expose an API to allow VPNs app to ask for running the validation
check on the VPN network built from Ikev2VpnProfile.

Bug: 184750836
Test: New test in Ikev2VpnTest
      Also FrameworksNetTests
Change-Id: I385bb887b6c697d8f5d87af750dbd2aab44afca6
CTS-Coverage-Bug: 184750836
 (but CTS is in the same topic, just not detected by the tool)