Add a note to generated plugins files (#45557)

There has been some confusion about whether or not
.flutter-plugins-dependencies should be tracked in version control or
not. Added a comment to both it and .flutter-plugins explaining that
it's generated and shouldn't be.

.flutter-plugins-dependencies is parsed through JSON, and the JSON spec
doesn't support comments. So unfortunately the note is living in an
arbitrary "_info" key instead of an obvious top level comment.