The PlatformView does not have a semantics container when added to semantics tree, this PR gives it a semantics container to ensure accessibility traversal works.
This fixes https://github.com/flutter/flutter/issues/135504, which is a regression of 738b1ad94d
Before 738b1ad94d, the traversal works because the PlatformView is added to the accessibilityElements of the FlutterPlatFormViewSemanticsContainer, which implicitly made the FlutterPlatFormViewSemanticsContainer as the PlatformVIew's AccessibilityContainer.
Now we use the PlatformVIew as the nativeAccessibility of the FlutterPlatFormViewSemanticsContainer, we need to expilicitly set the container.
This needs to be cherry-picked since the commit caused the regression was cherry-picked in 738b1ad94d
[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style
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