This commit adds the `loadData` and `loadDataWithBaseUrl` methods to the
pigeon communication interface. Next step would be to include the
implementation details on the Android (JAVA) side.

Adds the native Android implementation for the `loadData` and
`loadDataWithBaseUrl` API methods.

…_android

Adds the Dart implementation of Android's WebView `loadData` and
`loadDataWithBaseUrl` methods.

Also fixed support for null parameters in the native Android
implementation.

…_android

…_appfacing

Adds implementation to the `WebViewController` class to expose the
`WebViewPlatformController.loadFile` and
`WebViewPlatformController.loadHtmlString` methods.

Adds unit tests for the `loadFile` and `loadHtmlString` methods which
are added to the `WebViewController` class in commit
f8e9c32.

Adds example implementation of the `WebViewController.loadHtmlString`
and `WebViewController.loadFile` methods to the example app.

Updates the version of the app facing plugin to 2.4.0 and adds a
matching entry to the CHANGELOG.md.

…_appfacing