Make IpSecManager.DIRECTION_FWD @SystemApi

IpSecManager.DIRECTION_FWD is used by VcnGatewayConnection and
it will be not visible to platform after IpSec was moved into
Connectivity mainline module. Therefore, make DIRECTION_FWD
@SystemApi so that it can be used by VcnGatewayConnection.

Bug: 204153604
Test: FrameworksNetTests
Change-Id: I80dc2a5b4940504820cee0d8e2d2e59ef3fcca18