Add global setting to use new DHCPv4 server

The setting applies to the choice of DHCPv4 server for tethering.

Bug: b/109584964
Test: Still builds, SettingsBackupTest passes
Change-Id: Id9b5820883eaf3148f09ad948c34795b3619a30e
diff --git a/core/tests/coretests/src/android/provider/SettingsBackupTest.java b/core/tests/coretests/src/android/provider/SettingsBackupTest.java
index dea3235..cd96583 100644
--- a/core/tests/coretests/src/android/provider/SettingsBackupTest.java
+++ b/core/tests/coretests/src/android/provider/SettingsBackupTest.java
@@ -422,6 +422,7 @@
                     Settings.Global.TETHER_DUN_REQUIRED,
                     Settings.Global.TETHER_OFFLOAD_DISABLED,
                     Settings.Global.TETHER_SUPPORTED,
+                    Settings.Global.TETHER_ENABLE_LEGACY_DHCP_SERVER,
                     Settings.Global.TEXT_CLASSIFIER_CONSTANTS,
                     Settings.Global.THEATER_MODE_ON,
                     Settings.Global.TIME_ONLY_MODE_CONSTANTS,