Define the IpSecTransformState API flag in Connectivity module

The flag com.android.net.flags.ipsec_transform_state gates APIs
exposed by the Tethering module, and thus should also be included
in Connectivity/common/flags.aconfig

Bug: 324278950
Test: make
Change-Id: Ia1fe733a4971ac56cae65870a5339362594322e6