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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

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] 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].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

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---------

Co-authored-by: Matthew Kosarek <[email protected]>
Co-authored-by: Harlen Batagelo <[email protected]>
Co-authored-by: Loïc Sharma <[email protected]>