Remove the 'app' domain from flutter daemon (#18873)

* Remove the 'app' domain from flutter daemon

By default the daemon won't register the "app" domain, you need to opt-in (which the 'run' command does, as well as the tests for the app functionality).

Fixes #6658.

* Tweak text

* Put restart/callServiceExtension/stop back into daemon mode

* Add a comment about removing discoverApps