miscellaneous updates to the current_results_ui codebase

[devoncarew]

Various and sundry updates to the current_results_ui/ codease:

• use the url_launcher plugin; this lets use remove the dart:html references from the flutter app
• use package:flutter_lints for linting; this are the latest best practice lints for flutter. most of the code changes in this commit are mechanical changes performed by dart fix
• avoid using package:clippy; this was not migrated to null safety, and there's an API to do what we need in flutter
• delete some unused generated protobuf files
• migrate current_results_ui/ to null safety; all the changes here are what were suggested by dart migrate

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I see this in the readme.md:

The repository is based at dart.googlesource.com/dart_ci. It is mirrored to github.com/dart-lang/dart_ci. Do not land pull requests on Github.

If PR won't work here, I can recreate this work as a gerrit CL. However, it would be nice to either take PRs, or, add some brief contributing instructions to the repo to describe how to author changes.

You might consider flipping the system of record for the codebase, as that would allow us to do things like use github actions to validate the code.

[devoncarew]

Thanks for moving this to a gerrit review! I lgtm'd it there.