* Move the handling of delegating AppDelegate callback out of FlutterAppDelegate.

Also moves the plugin registry to FlutterViewController. So each view-controller will handle its
own plugins.

This is intended to simplify including one or more Flutter views in an existing iOS app and giving
more precise control of plugin registration.

Fixes: https://github.com/flutter/flutter/issues/16539

* formatting

* Update license golden file

* Fixed type error

* FREEZE.unindexed

* Fix Header types

* Revert "FREEZE.unindexed"

This reverts commit bebb70056c9bcb90b4321bdc2873896623ed6faa.