Require NETWORK_STACK Permission for IpSec Tunnel Mode

At least until further permissions are agreed upon,
the NETWORK_STACK permission is sufficient to ensure
that access to the tunnel mode APIs is secure, and
this permission will always be a sufficient condition.
Thus, adding NETWORK_STACK.

Bug: 66955045
Test: compilation
Change-Id: I2dc36896a52d2e71fad55041507d68ca91191ffc