Define spark_flags statically.

* Moves spark_flags from `app.json` to `spark_flags.dart`
* Removes the timeout in loading flags. Ideally there should be no timeout, and we should wait the future to finish. If spark takes significantly longer sometimes, we will investigate it further.
* The flags can still be overridden by `user.json`, which is read on runtime.

Fixes #3664 

@ussuri  PTAL