Improves Windows's lifecycle event for window destruction:

1. Made FlutterWindowsView generate the "window hidden" event when the HWND is destroyed instead of the FlutterWindow.
   1. Before the window would submit this event using a destroyed view:
      1. The FlutterWindowsView destructor runs to completion.
      2. The view owns the window, so the FlutterWindow is destroyed
      3. The window's destructor generates a "window hidden" event
      4. The window uses the destroyed view to notify the engine of the event
2. Adds an app lifecycle integration test to verify the resumed and hidden events are sent when an app is launched and closed.
3. Removed the FlutterWindowTest type as it was unused

Prepares for flutter/flutter#137267
Prepares for flutter/flutter#142845

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 Flutter Style Guide and the C++, Objective-C, Java style guides.
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or the PR is test-exempt. See testing the engine for instructions on writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
• I signed the CLA.
• All existing and new tests are passing.

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