Build fails on Android with SDK 33

Building on Android with `compileSDKVersion 33` fails due to using a deprecated API:

```
flutter/.pub-cache/hosted/pub.dartlang.org/plain_notification_token-0.0.4/android/src/main/java/net/kikuchy/plain_notification_token/PlainNotificationTokenPlugin.java 
uses or overrides a deprecated API.
```