[webview_flutter] Android implementation of V4 platform interface

[mvanbeusekom]

Android implementation of the version 4 of the webview_flutter_platform_interface. To make it easier to review this PR I added functionality in order:

• [x] Implementation of the AndroidWebViewCookieManager (commit 0929348).
• [x] Implementation of the AndroidWebViewJavaScriptChannel (commit c09b215).
• [x] Implementation of the AndroidNavigationDelegate (commit 60dfde1).
• [ ] Implementation of the AndroidWebViewController (commit ...).
• [ ] Implementation of the AndroidWebViewWidget (commit ...).
• [ ] Update the example App to use v4 implementation (commit ...).

Fixes Android part of flutter/flutter#94051

If you had to change anything in the flutter/tests repo, include a link to the migration guide as per the breaking change policy.

Pre-launch Checklist

• [x] I read the Contributor Guide and followed the process outlined there for submitting PRs.
• [x] I read the Tree Hygiene wiki page, which explains my responsibilities.
• [x] I read and followed the relevant style guides and ran the auto-formatter. (Unlike the flutter/flutter repo, the flutter/plugins repo does use dart format.)
• [x] I signed the CLA.
• [x] The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• [x] I listed at least one issue that this PR fixes in the description above.
• [x] I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is exempt from version changes.
• [x] I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style.
• [ ] I updated/added relevant documentation (doc comments with ///).
• [ ] I added new tests to check the change I am making, or this PR is test-exempt.
• [ ] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.