Aggressively apply the const lint.

* Rename platform_services to platform_channel

* rename in README

* renamed tests

* update modules.xml

Changed standard encoding to use host endianness. Engine roll.

* Add support for earlier Android versions

* Addressed comments

Fix a few regressions.

New concepts: PlatformMessageChannel (basic message send/receive superseding some existing  PlatformMessages methods), PlatformMethodChannel (method invocation and event streams), pluggable codecs for messages and method calls: unencoded binary, string, json, and 'standard' flutter binary encoding.

* 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 class is just an alias for PlatformMessages. We'll remove it after a short
deprecation period.

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.

This example shows how to build a basic app using a custom Xcode project.
Future patches will bring the functionality up to par with hello_android.

Now you can import 'package:sky/widgets.dart' to get all the widgets.

Converted demo launcher example to library

Converted fitness example to library

Converted hello world example to library

Converted mine digger example to library

Converted stocks example to library