Don't try to use `WebAppInternals.refreshableAssets` until it's ready.

`refreshableAssets` gets set from the `generateBoilerplate` call in
webapp's `main` function. `updateVersions` in autoupdate gets called in
a startup hook, which runs before `main`. So we need to wait until
webapp is "more ready" (i.e. `onListening`) to try to use
`WebAppInternals.refreshableAssets`.