Tidy up VPN code

This commit addressed the comments in different commits, it did
- Make isVpnApp() to be a static method.
- Rename getRedactedNetworkCapabilitiesOfUnderlyingNetwork to
  getRedactedNetworkCapabilities and rename
  getRedactedLinkPropertiesOfUnderlyingNetwork to
  getRedactedLinkProperties.
- Reduce the scope of synchronized block in handleSessionLost().
- Fast return when there is an IllegalArgumentException in
  handleSessionLost().
- Remove inaccurate comment.
- Update the document of mCurrentToken.
- Define an initial token value of IKE session.
- Add "/* exeception */" annotation to handleSessionLost(null)
  calls.

Bug: 236409954
Test: atest FrameworksNetTests:VpnTest
Change-Id: I8c938f8796345dc2eee6c8eff513b0f993bc8165