[ViewModel] Implement UIAccessibilityTraitButton

[CharlesMcD]

When using a simple ViewModel instead of a ButtonViewModel, we don't see any of the buttons being described as "button", because the view model is bound to plain views. One way to fix this is to force the accessibility role or trait.

On iOS, you simply add the UIAccessibilityTraitButton trait to the accessibilityTraits value: view.accessibilityTraits |= .button

On Android, it's a bit more complicated, you have to use an accessibilityDelegate to force the class name, or override view.onInitializeAccessibilityNodeInfo

ViewCompat.setAccessibilityDelegate(model, object : AccessibilityDelegateCompat() {
    override fun onInitializeAccessibilityNodeInfo(host: View, info: AccessibilityNodeInfoCompat) {
        super.onInitializeAccessibilityNodeInfo(host, info)
        info.className = android.widget.Button::class.java.name
    }

This wouldn't be necessary it the button view model were attached to actual button views.

Thanks @dominiclabbe for the proposition!