Add app activation hook to Flutter embedder

To allow switching between multiple applications from the Flutter homescreen, a mechanism is needed to call the agl-shell protocol's "activate_app" method.  Since the Flutter embedder is the agl-shell client and the required wayland connection is internal to it, the most straightforward seeming way to implement this in the near term is to expose the method via a simple platform channel plugin.  This issue will be used to track the required changes.