Do not remove policy levels when traffic statistics is disabled (#6146)

Disabling traffic statistics removed the whole policy section from the
generated config, including the levels block (uplinkOnly, downlinkOnly,
connIdle, handshake) from the template or from a user's custom config.
Only the stats-related policy.system block should be stripped; keep
policy.levels intact in both the unified and the custom config paths.

V2rayNG/app/src/main/java/com/v2ray/ang/core/CoreConfigManager.kt

@@ -113,8 +113,14 @@ object CoreConfigManager {
             }
         } else {
             json.remove("stats")
+            // Keep user-defined policy levels, only strip the stats-related system block
+            json.get("policy")?.takeIf { it.isJsonObject }?.asJsonObject?.let { policy ->
+                policy.remove("system")
+                if (policy.entrySet().isEmpty()) {
                     json.remove("policy")
                 }
+            }
+        }
 
         if (!needTun()) {
             return JsonUtil.toJsonPretty(json)?.let { ConfigResult(true, configContext.guid, it) } ?: result
@@ -709,7 +715,7 @@ object CoreConfigManager {
     private fun applySpeedDisabled(v2rayConfig: V2rayConfig) {
         if (MmkvManager.decodeSettingsBool(AppConfig.PREF_SPEED_ENABLED) != true) {
             v2rayConfig.stats = null
-            v2rayConfig.policy = null
+            v2rayConfig.policy?.system = null
         }
     }