Enable MOBIKE in IKEv2 VPN

This commit enables MOBIKE on the VPN's IKE Session. In this way if
the VPN server also supports MOBIKE, IKEv2 VPN can migrate to a
different network by migrating the active IKE session, instead of
setting up a new IKE Session.

This commit also adds a grace period so that when the underlying network
is lost, the IKEv2 VPN will wait for some time before terminating the
IKE Session in case a new network will be available.

Bug: 192077544
Test: atest IkeV2VpnTest (new tests added)
Test: atest com.android.server.connectivity.VpnTest
Change-Id: Ie6ef3103200fef3b7e5f00918fee9e791cad9445