Re-write memory_service_test for Flutter app

[kenzieschmoll]

memory_service_test currently uses memory_controller.dart and memory_protocol.dart from the html app files. The test needs to be rewritten to use the files from the flutter app.

[kenzieschmoll]

Test disabled in #2052

[srawlins]

Closing as obsolete: The legacy HTML web app and its associated tests were removed, and the Memory screen now has a dedicated Flutter test suite under packages/devtools_app/test/screens/memory/.

If there is more work to do here, please let us know by filing a new issue with up to date information. Thanks!