See #143782, #143782 (comment).

As-per chatting with @zanderso let's do the following:

1. Pull out the clause from lint.sh to ban_generated_plugin_registrant_java.sh
2. Create a new invocation of the script on linux_unopt

In addition, we should make the following improvements (/cc @dnfield):

1. Only look for files in src/flutter (i.e. it should not look in src/third_party)
2. Additionally ignore src/flutter/third_party for the same reason (eventually they will be the same)

I considered moving this to a Dart script, but I'm concerned about the performance cost of using a deep recursive file search versus just shelling out to find, and because this mostly breaks local development it's nice to get actionable feedback from the git hooks instead of waiting purely for CI failures.