feat: web based applications will now flush events before closing to ensure events are sent

[tanderson-ld]

Requirements

• [x] I have added test coverage for new or changed functionality

• [x] I have followed the repository's pull request submission guidelines

• [x] I have validated my changes against all supported platform versions

Describe the solution you've provided

Created LDAppLifecycleListener and configured to use conditional imports. In the web case, uses an LDAppLifecycleListener that uses html document visibility. In the non-web case, uses the existing Flutter AppLifecycleListener. Note that the web based LDAppLifecycleListener only emits resume and hidden, no other events.

[shortcut-integration[bot]]

This pull request has been linked to Shortcut Story #232926: Flutter SDK should attempt to flush events, in the browser, when it detects signals a page will close..