Allowing access to FlutterFragmentActivity's content view ID.

Currently this is the way for user to access root layout ID (FRAGMENT_CONTAINER_ID) when extending from FlutterFragmentActivity, but what if there were breaking changes?

private fun getRootViewId(): Int {
var rootView: ViewGroup? = (this.findViewById(android.R.id.content) as ViewGroup).getChildAt(0) as ViewGroup
return rootView?.getId() ?: NO_ROOT_VIEW
}

Activities extending from FlutterFragmentActivity can add views to root layout instead of creating a new layout.
This is especially useful when the user wants to create additional fragments and transact them to the root layout.

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