Minor cleanups in VPN

This commit includes few minor cleanups in VPN for better readability
and maintainability. Specifically this commit:
- explicitly sets IPsec tunnel's underlying network when Child SA is
   created
- nulls out ScheduledFuture when the task is finished or cancelled
- validates the IKE Session token before executing the scheduled
  handleSessionLost call

Bug: 192077544
Test: atest VpnTest, IkeV2VpnTest
Change-Id: Ib3cbdbfa594c55c27b78dffc00a82d371ca7a749