service: Make the entrypoint a VM entrypoint

Should help with Flutter optimizing the function away.
Based on
https://github.com/ekasetiawans/flutter_background_service/commit/6c353dd4fc65818ffde3fdcd8364604ceaff569b.

lib/service/service.dart

@@ -147,6 +147,7 @@ Future<void> initUDPLogger() async {
 }
 
 /// The entrypoint for all platforms after the platform specific initilization is done.
+@pragma('vm:entry-point')
 Future<void> entrypoint() async {
   // Register the lock
   GetIt.I.registerSingleton<Completer<void>>(Completer());