Fixes #163104

The core issue is that EmbedderTaskRunner can schedule things in advance, but there is no checking if the task is stale when executing it. It is possible that FlutterEngineRunTask will attempt to run the task on engine that is non null but already being deallocated.

This in not a problem for raster and platform task runners, because raster task runner shouldn't have any scheduled tasks after shutdown and platform task runner executes the shutdown process, so the pointer is always either valid or null, but it is an issue for custom ui_task_runner, because it may be calling FlutterEngineRunTask with engine pointer that is not yet null but already shutting down.

The proposed solution is to assign a unique identifier for each EmbedderTaskRunner, use this identifier as the runner field inside FlutterTask instead of casting the address of the runner, and verify that this identifier is valid inside FlutterEngineRunTask before calling anything on the engine.

Special care needs to be done to not invalidate the unique identifier while ui_task_runner is running a task as it may lead to raciness. See EmbedderEngine::CollectThreadHost().

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, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• I followed the breaking change policy and added Data Driven Fixes where supported.
• All existing and new tests are passing.

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