See flutter/engine#3539

* Rename platform_services to platform_channel

* rename in README

* renamed tests

* update modules.xml

Changed standard encoding to use host endianness. Engine roll.

* Add platform_services sample

* update build.gradle to use latest scripts

* use FlutterActivity

* Updated ExampleInstrumentedTest.java and deleted FlutterPlugin.groovy

* Remove getRandom code from main.dart

* remove unused import

This patch changes the default `flutter create` command to generate a
HelloServices-style app on iOS. As a consequence, the Atom integration now
fully works with HelloServices.

Cleans up a few style nits in hello_services and adds support for automatic
JSON encoding and decoding to the HostMessages interface.

Currently this service just returns mock data, but it exercises the plumbing.