Add name space settings_stats

settings_stats name space contains whitelist keys which is needed
for logging.

Design doc: https://docs.google.com/document/d/1OijtO3QfFxYiIUyNbVm7pGzZ8zx2xV-1CrLYQEIGDTI/edit
Bug: 142941011
Test: rebuild

Change-Id: Ib6cb2b1203dbc0f5c575099e91dd0a40a3e17ca9

core/java/android/provider/DeviceConfig.java

@@ -265,6 +265,13 @@ public final class DeviceConfig {
     @SystemApi
     public static final String NAMESPACE_SCHEDULER = "scheduler";
 
+    /**
+     * Namespace for settings statistics features.
+     *
+     * @hide
+     */
+    public static final String NAMESPACE_SETTINGS_STATS = "settings_stats";
+
     /**
      * Namespace for storage-related features.
      *