[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

Start by reproducing nested accessible touchables in a React Native iOS app with VoiceOver, then investigate the UIAccessibilityContainer protocol and the parent's accessibilityElements property described in the issue. Done means both the row and its nested save/delete button are selectable by VoiceOver, matching Android behavior.