Add getHitTestResult() functionality

There is a web page that in the page show a close button. So on android native we can detect if user click on webview and then using getHitTestResult() to check which element on the web that got clicked. It will be really useful if we have this feature.

Start at the Android WebView integration and the getHitTestResult() entry point mentioned in the issue. Determine how a WebView hit-test result should be exposed through the Flutter plugin, and define done as allowing Flutter code to identify the clicked web element, including the close-button case.

Written by the indexing model from the issue text.