Fix crash when closing a window with Alt+F4 in multi-win Flutter on Windows

[hbatagelo]

Fixes #158450.

As mentioned in #158450, the crash occurs because a destroyed object may be accessed if the window and view have already been destroyed by the time KeyEventCallback is called. This issue is not limited to the Alt+F4 system key; it may also occur if the window is closed using other key presses, such as pressing Enter after navigating to a dialog's "Close" button.

This PR proposes a fix that checks whether the view ID is still valid when the callback is invoked. If the view is invalid, the event is skipped for that view.

A unit test has been added to assert that the KeyEventCallback is invoked when the associated view is valid, and not invoked when the view is destroyed.

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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