Migrated xylophone-flutter stub project to support Flutter 2.2.3

[tigerjoy]

The stub project was migrated to mitigate the errors that were being displayed when was used a template to build Flutter projects.

The migration instructions were followed from https://github.com/flutter/flutter/wiki/Upgrading-pre-1.12-Android-projects

This stub was tested to be running on an Android Emulator.

[vialcollet]

Here is a set of instructions to pull this:

git clone https://github.com/londonappbrewery/xylophone-flutter.git
cd xylophone-flutter
git pull origin pull/36/head