Add a boolean flag to show or hide Private DNS in Network & internet.

The following boolean flag is added:
config_show_private_dns_settings

When set to false, Private DNS will not be shown in Network & internet.

Bug: 111398942
Test: make RunSettingsRoboTests
ROBOTEST_FILTER=PrivateDnsPreferenceControllerTest
Change-Id: I62911930680355b7fc109b295d8526bdb7eac0b3

src/com/android/settings/network/PrivateDnsPreferenceController.java

@@ -80,7 +80,9 @@ public class PrivateDnsPreferenceController extends BasePreferenceController
 
     @Override
     public int getAvailabilityStatus() {
-        return AVAILABLE;
+        return mContext.getResources().getBoolean(R.bool.config_show_private_dns_settings)
+                ? AVAILABLE
+                : UNSUPPORTED_ON_DEVICE;
     }
 
     @Override