[a11y] Nested accessibility items are not respected on iOS

React Native apps often have many UI components that are touchable but also contain nested elements that are also touchable. For example, an entire list row may be touchable as a primary navigation option, but it might also have an auxiliary button nested inside of it for taking a specific action, like delete or save.

Currently, if the touchable wrapper for the auxiliary button as well as the row container are properly tagged with accessibility props and accessibility labels it works fine on Android but on iOS, the nested 'save/delete' button is not selectable by VoiceOver. On Android, both will be accessible when accessibility features are enabled.

The current workaround in user-land is to make some janky hierachry where the aux button is a sibling of its visual container, which often breaks encapsulation and is generally a pain.

There are a couple ways we might be able to work around this at the framework level:

1. use UIAccessibilityContainer protocol
2. set the accessibilityElements property on the parent

Those approaches are detailed a bit more here:
https://stackoverflow.com/questions/38849389/voiceover-parent-and-child-views-as-accessibility-elements

Looks like flutter had this same issue and fixed it here with UIAccessibilityContainer: https://github.com/flutter/engine/pull/4110

Tracked internally with T34121499

首先，在使用 VoiceOver 的 React Native iOS 应用中复现嵌套的可访问 touchable，然后研究 UIAccessibilityContainer 协议以及 issue 中描述的父级 accessibilityElements 属性。当行及其嵌套的保存/删除按钮都可以通过 VoiceOver 选择，并且与 Android 的行为一致时，即表示完成。