Add hidden API getDataSaverViaCmFlag() for CTS

The CtsHostsideNetworkTests:com.android.cts.net.
HostsideNetworkCallbackTests needs to know whether trunk flag is enabled
or not. But there is no stable way to read the flag from CTS for module.
So adds this hidden API in platform.

Bug: 288340533
Test: atest
CtsHostsideNetworkTests: com.android.cts.net.HostsideNetworkCallbackTests

Change-Id: I02cea917a3f75f11d6ea5754a6ea0d460f1372c9

core/java/android/net/NetworkStack.java

@@ -23,6 +23,7 @@ import android.content.Context;
 import android.os.IBinder;
 import android.os.ServiceManager;
 
+import com.android.net.flags.Flags;
 import com.android.net.module.util.PermissionUtils;
 /**
  * Constants and utilities for client code communicating with the network stack service.
@@ -103,4 +104,16 @@ public class NetworkStack {
             final @NonNull String... otherPermissions) {
         PermissionUtils.enforceNetworkStackPermissionOr(context, otherPermissions);
     }
+
+    /**
+     * Get setting of the "set_data_saver_via_cm" flag.
+     *
+     * @hide
+     */
+    // A workaround for aconfig. Currently, aconfig value read from platform and mainline code can
+    // be inconsistent. To avoid the problem, CTS for mainline code can get the flag value by this
+    // method.
+    public static boolean getDataSaverViaCmFlag() {
+        return Flags.setDataSaverViaCm();
+    }
 }