Prevent migrating the session from using the wrong value

Migrating the IKE session requires reading the subId information
from network capabilities. A new default network change will
start with an onAailable callback and start the migration. But
Vpn does not yet get the new network capabilities. The migration
should not be processed until receiving network capabilities
change.

Bug: 269714970
Test: atest FrameworksNetTests
Change-Id: I1d2eb9647cd3a0434ba656671926eb504abbf150