Apply transform to FWD policy if configured to provide tethering

This change allows the VCN to trigger the application of IpSecTransforms
as in the FWD path, ensuring that tethering works across interfaces

Bug: 185495453
Test: atest FrameworksVcnTests
Test: manual testing with cross-device tethering
Change-Id: I90fc77226ca971dbd8ac549b96a4828da1079cd5