[select] Decouple Select styles from NativeSelect

SelectInput reused the styled select and icon from NativeSelectInput,
so NativeSelect carried the open icon rotation and the iconOpen class
even though a native select never reports being open, and
MuiNativeSelect theme variants also styled the Select display and icon.

Give SelectInput its own styles, drop the open handling from
NativeSelectInput and document that NativeSelect never applies iconOpen.

docs/translations/api-docs/native-select/native-select.json

@@ -48,9 +48,7 @@
       "conditions": "<code>variant=\"filled\"</code>"
     },
     "iconOpen": {
-      "description": "Styles applied to {{nodeName}} if {{conditions}}.",
-      "nodeName": "the icon component",
-      "conditions": "the popup is open"
+      "description": "This class is never applied, a native select doesn&#39;t expose whether its popup is open."
     },
     "iconOutlined": {
       "description": "Styles applied to {{nodeName}} if {{conditions}}.",