style:optimize UI logic

1.add button for cancelling delete server config
2.toast when delete current using config

V2rayNG/app/src/main/res/values-zh-rCN/strings.xml

@@ -80,6 +80,7 @@
     <string name="toast_asset_copy_failed">失败, 请使用文件管理器</string>
     <string name="menu_item_add_file">添加文件</string>
     <string name="menu_item_download_file">下载文件</string>
+    <string name="toast_action_not_allowed">禁止此项操作</string>
 
 
     <!-- PerAppProxyActivity -->