Add explicit override for default Private DNS mode

Test: as follows
    - built
    - flashed
    - booted
    - runtest frameworks-net passes
Bug: 79719289
Change-Id: I943c5476666e47d04690626e2133f501cb875b46
diff --git a/core/tests/coretests/src/android/provider/SettingsBackupTest.java b/core/tests/coretests/src/android/provider/SettingsBackupTest.java
index 22aca30..5b7fc6e 100644
--- a/core/tests/coretests/src/android/provider/SettingsBackupTest.java
+++ b/core/tests/coretests/src/android/provider/SettingsBackupTest.java
@@ -358,6 +358,7 @@
                     Settings.Global.PREFERRED_NETWORK_MODE,
                     Settings.Global.PRIV_APP_OOB_ENABLED,
                     Settings.Global.PRIV_APP_OOB_LIST,
+                    Settings.Global.PRIVATE_DNS_DEFAULT_MODE,
                     Settings.Global.PROVISIONING_APN_ALARM_DELAY_IN_MS,
                     Settings.Global.RADIO_BLUETOOTH,
                     Settings.Global.RADIO_CELL,