add asset by url menu, ui and save logic

V2rayNG/app/src/main/kotlin/com/v2ray/ang/util/MmkvManager.kt

@@ -12,6 +12,7 @@ object MmkvManager {
     const val ID_SERVER_RAW = "SERVER_RAW"
     const val ID_SERVER_AFF = "SERVER_AFF"
     const val ID_SUB = "SUB"
+    const val ID_ASSET = "ASSET"
     const val ID_SETTING = "SETTING"
     const val KEY_SELECTED_SERVER = "SELECTED_SERVER"
     const val KEY_ANG_CONFIGS = "ANG_CONFIGS"