[webview_flutter] Support new onUrlChanged event on Android.

[BeMacized]

This PR adds Android support for the new onUrlChanged event added to the platform interface in #4509.

As this PR contains and depends on the changes made to the platform interface in #4509, this PR will remain in draft state until that PR has been successfully merged and the changes have been published.

Relevant issue:

• WebView for Flutter - support listen and reacting to URL changes flutter#27729

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• 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.)
• I signed the [CLA].
• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• I listed at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the [pub versioning philosophy], or this PR is [exempt from version changes].
• I updated CHANGELOG.md to add a description of the change.
• 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.

[Hixie]

@BeMacized are you still interested in working on this PR? Is there anything you need from anyone?

[BeMacized]

@Hixie We stopped working on this feature due to there being no good way to implement it in the current structure without a breaking change. The changes to the platform interface these PRs depend on that were merged have been reverted. Once #94051 has been resolved I'll be happy to pick it back up again.

[Hixie]

@BeMacized should we close this PR in the meantime? We can always refer to the changes in a closed PR. (I ask just because this is one of our oldest PRs and I'm generally trying to clean up our PR queue.)

[BeMacized]

@BeMacized should we close this PR in the meantime? We can always refer to the changes in a closed PR. (I ask just because this is one of our oldest PRs and I'm generally trying to clean up our PR queue.)

Yes, I'm closing this. I'll probably be rewriting this feature once 4.0 has been released.