A PR to add support for `accessibilityViewIsModal` in `CollectAccessibilityElements`.

If in a list of subnodes more than 1 subnode has `accessibilityViewIsModal` marked as `YES`, then the node with the highest index in `subnodes` will be the one that is considered modal. This behavior matches UIKit.

If the value of `accessibilityViewIsModal` changes, we need to clear all the cached `accessibilityElements` from that view up. I added this in ASDisplayNode’s `setAccessibilityViewIsModal` method. Note that if we ship `ASExperimentalDoNotCacheAccessibilityElements` then we can remove the invalidation step.

Finally, I changed all the tests to ask the view for accessibilityElements, not the node. This is a better representation of what will really happen when UIKit asks a node’s view for its accessibility elements. It also allowed me to test that clearing the accessibilityElements was working.