Is WebView localStorage stored in the app cache?
The app cache helps the app run faster. Every time an app loads some data (like images and so on) it gets stored in the app cache. If the user opens the app again, this data is not reloaded. It is taken from the app cache instead. This makes the app faster. App cache though does not persist and is erased every time you restart your phone.

In the localStorage though, you store things that the app will need to remember even when the phone restarts. For example what is your progress in a game, some app parameters etc.

See more about app cache here.