Currently the constructor of the Java FlutterWebView is populated with several lines of code that initialise and configure the Android WebView control. This has two major drawback:

1. The code becomes difficult to understand;
2. The code is extremely hard to test as there is no good way to mock the WebView control.

This PR solves this problem by introducing a WebViewBuilder which is responsible for correctly creating the right Android WebView control and applying the necessary configuration.

The following PRs will directly benefit from this change:

• [webview_flutter]Add the loadRequest method #4169
• [webview_flutter] WIP Webview transparent background #4165

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

• 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. (Note that 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.
  • No release needed as nothing changed in the behaviour of the plugin. Simply a refactor of code structure to make the code easier to test for later features (see PRs who will benefit).
• I updated CHANGELOG.md to add a description of the change.
  • No release needed as nothing changed in the behaviour of the plugin. Simply a refactor of code structure to make the code easier to test for later features (see PRs who will benefit).
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test exempt.
• All existing and new tests are passing.

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