Add a feature flag for the new tether settings UI

The flag is mirrored from FeatureFlagUtils in framework/base
Bug: 148182953
Bug: 148220934
Test: Build successfully.
Change-Id: Ifada88cb585b3f618b7e2f59f1ceff007ae8eacd
diff --git a/src/com/android/settings/core/FeatureFlags.java b/src/com/android/settings/core/FeatureFlags.java
index 3321c05..d2ad30d 100644
--- a/src/com/android/settings/core/FeatureFlags.java
+++ b/src/com/android/settings/core/FeatureFlags.java
@@ -28,4 +28,5 @@
             "settings_wifi_details_datausage_header";
     public static final String CONTROLLER_ENHANCEMENT = "settings_controller_loading_enhancement";
     public static final String CONDITIONAL_CARDS = "settings_conditionals";
+    public static final String TETHER_ALL_IN_ONE = "settings_tether_all_in_one";
 }