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

common/flags.aconfig

@@ -39,3 +39,10 @@ flag {
   bug: "294777050"
 }
 
+flag {
+  name: "ipsec_transform_state"
+  namespace: "android_core_networking_ipsec"
+  description: "The flag controls the access for getIpSecTransformState and IpSecTransformState"
+  bug: "308011229"
+}
+