[CP-beta][iPhone Duo] Use the correct UIScreen to report the display size and update when fold/unfold - #195690

…update when fold/unfold (flutter#195199)

On iPhone Duo, `UIScreen.mainScreen` always refers to the outer display.
The iOS embedder used `UIScreen.mainScreen`, so when the app is on the
inner display, `PlatformDispatcher.displays` reports the wrong screen.

This PR ensures the embedder to report the correct screen size to the
framework, and when switch between inner/outer screens, update the
framework with the new size.

## Why still need `UIScreen.mainScreen` fallback

The framework can read display information as soon as the isolate
launches, before any view is in a window (see this test project
flutter#195177 (comment)).
At that point we do not know which screen the FlutterVC will be added to
(if we ever add it to the screen).

MainScreen remains a best-effort initial guess (better than a `.zero`
size since flutter developers may have already depend on the value). It
is corrected as soon as the view is laid out in a window.

`_hasSentDisplays` makes the engine use the `UIScreen.mainScreen`
fallback only for the very first display it sends, before any view is in
a window. After that, if the view isn't in a window (e.g. a newly
attached view controller whose view isn't loaded yet), the engine keeps
the last reported display instead of replacing it with the `mainScreen`
guess. The flag resets when a new shell is adopted, so each new shell
still gets an initial display.

## Why not fallback based on hinge state? 

1. Hinge API only works for a view that's already in a window. 
2. There's no dedicated API to get the inner display (unless with
another deprecated `UIScreen.screens` API which isn't great either).
3. Open state could use both inner and outer displays (accessory scene)

## Why change in `SemanticsObject` is simpler than `FlutterEngine/VC`

Because `SemanticsObject` is a "pull" model, where we simply need to
check which screen when accessing it. On the other hand,
`FlutterEngine/VC` is a "push" model, where we have to make sure to
update the value when screen changes.

## Integration test

I don't think there's a `simctl` sub-command that allows us to
fold/unfold the simulator. We could add a integration test in the future
if such api become available.

## Manual test

I have manually verified using [this test
project](flutter#195177 (comment)),
and confirmed the reported value is correct, and updated correctly once
fold/unfold

Scenario | Window screen | Before (stock engine) | After (this PR)
-- | -- | -- | --
Launch closed | 1398x2034 | 1398x2034 | 1398x2034
Closed → open | 2007x2853 | ❌ 1398x2034 | ✅ 2007x2853
Open → closed | 1398x2034 | 1398x2034 | ✅ 1398x2034
Launch open (first read in main()) | 2007x2853 | ❌ 1398x2034 for the
whole session | ✅ 2007x2853



Working towards flutter#195177

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