The Android webview should always be "enabled" -- I'm not sure what you mean by this. I've never seen a device where you cannot install and run a Cordova app (which is what the XDK creates). All Cordova apps use the native webview unless you build with Crosswalk, in which case the app uses the Crosswalk webview.
The native webview in Android 5 and later devices is updated through the store, but there is no way for your app to know if an update is available, all you can do is detect if the webview is compatible with your app.
So I think what you really want to do is check the webview runtime version and decide if your app will work or will not work. If your app will not work with the version of the webview that your app is running on, you should show a screen indicating that your app is not compatible with this device. If you need a guarantee that the webview will be a specific level you should be building with Crosswalk embedded, that way you can guarantee that the same webview is used for every installation.
