When updating buttons with mergeOptions, existing custom buttons are unmounted and then recreated. This commit adds an optimisation which prevents the react components from being recreated.
To avoid unnecessary recreation, add an `id` to the component. If an existing react view is found for that id, it will be reused.

* Fix button with react component doesn't get unmounted

* Test e2e only on iOS

* v2-customButtons

* Update RNNRootViewController.m