The Android and iOS versions of this function had diverged. This patch
makes the iOS version match the Android version (and what the framework
expects).

Fixes https://github.com/flutter/flutter/issues/8878

Breaking change: removed facilities for JSON and string messaging from FlutterView/FlutterViewController, leaving only binary messaging there. All other use of flutter communication now goes through FlutterMessageChannel and FlutterMethodChannels. Retained use of String and JSON codecs for now.

Companion flutter PR: flutter/flutter#8837

This will eventually replace the clipboard mojom.

This method will let us remove the activity service.

The platform use cases are handled by the PlatformPlugin. The media service and
the sensor service have no clients. If we want to use them, we should create
plugins for them.

Also, expose a convenience class for processing JSON messages.