appview: follow web app manifest standard
- `id` member in webmanifest should be url format based on spec.
- it is recommended to use `application/mniafest+json` mimetype
- changing the name from `pwa-manifest.json` to `manifest.webmanifest`
Reference: <https://www.w3.org/TR/appmanifest/>
Signed-off-by: Seongmin Lee <git@boltless.me>