Improvements to M3 demo app (#1647)
* improvements
* fix focus
* add comment
* add comment
* copy changes to root material_3_demo
* fix large breakpoint
* fix large breakpoint
* Create integration_test.dart
* refactor main.dart into home.dart and constants.dart
* add integration_test to pubspec
* copy to root material_3_demo
* remove removal of constraints
* address feedback
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