DevTools License and 3P Notice Review

[mossmana]

• Conduct a review of all code in DevTools and determine where licenses need to be added/updated and/or require a 3P notice for dependencies.
• Write a utility that can test for missing/incorrect licenses and add/update the licenses where necessary.

// Copyright 2019 The Flutter Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file or at https://developers.google.com/open-source/licenses/bsd.

• Update DevTools tool to execute the utility from the command line. (NOTE: This will only be available from the forked branch unless there is enough interest to merge into the main repo.)
• Run the DevTools tool command against the repository files.
• DevTools should also show notices in an about dialog or help menu option.

e.g. Here's what the help menu option looks like in VS Code (just a link that redirects to https://code.visualstudio.com/license?lang=en)