Fixes UI freezes when multiple Flutter VC shared one engine

[zhongwuzw]

Description

Assume we have two Tabs, A、B, both share the same engine, when we switch Tab from A to B, A's viewDidDisappear is called after B's viewWillAppear, so it break the surface creation.

Related Issues

flutter/flutter#39036 (comment)

Tests

Not yet.

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[gaaclarke]

Looks good, please add a test here: https://github.com/flutter/engine/blob/master/shell/platform/darwin/ios/framework/Source/FlutterViewControllerTest.mm

[xster]

Thanks for your contribution. Please see https://github.com/flutter/flutter/wiki/Testing-the-engine#objective-c---ios-embedding for how to add a test.

[zhongwuzw]

@gaaclarke @xster Done.

[gaaclarke]

I updated your tests. If you add a negative test you should have a positive test too.

[xster]

LGTM

[xster]

Can we tweak the names a bit? Sounds like we're testing for 2 different outcomes with the same conditions :)

[gaaclarke]

i'll follow up with a PR so we can land this now.