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