Use token to identify IKE Session

This commit updates VPN to use an integer token to identify an IKE
Session.

This is a preparation commit to enable IKE mobility where an IKE
Session will be able to migrate to a different network and thus network
can no longer identify an IKE Session.

Bug: 192077544
Test: atest IkeV2VpnTest
Test: atest com.android.server.connectivity.VpnTest
Change-Id: I18d9a049587d7dfc05e2143320431014d053abbc