Address review comments on PR #24539

[bzbarsky-apple]

1. Instead of codegen-cpp-only do something like --generator codegen --codegen cpp-app
2. Instead of messing with the filter, just avoid adding CodegenJavaPregenerator to the list if codegen is cpp-app.

[andy31415]

We may also just want to extend the --generator format to standardize on some separator. Like accept codegen but also codegen:java or codegen:cpp-app to control the internal generator list.

In the PR review I am basically asking to not special case cpp-app but make it more generic (and also control generator array instead of passing down this to accept methods since the intent seems to control the generator)