Required for multi-window.

On windows the LifecycleManager currently sends the lifecycle event as soon as windows message is processed. This however causes problems when changing focus between application windows.

When switching focus from HWND1 to HWND2, HWND1 first gets unfocused, followed by HWND2 getting focused. After HWND1 gets unfocused, LifecycleManager immediately notifies the framework that the application is inactive, which is wrong as the application never went into inactive state, followed by subsequent call to put the application in resumed state when HWND2 is focused. Because this happens very quickly, sometimes focus manager gets into inconsistent state.

To resolve this LifecycleManager should gather the all the changes while window sends the messages and then notify the framework atomically in next run loop turn.

This PR also simplifies the logic in LifecycleManager through which the application state is derived from window states.

This PR removes engine forcing resumed lifecycle state at startup. I'm not entirely sure what the point of this was - the state can and should be determined solely from window states, this just seems to muddy the state logic. Also it happens before the framework is even listening to state changes.

The mutex in WindowsLifecycleManager is removed. Not sure why it was there.

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 ///).
• [x ] 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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