Implement category tile expansion with translucency and scroll beneath

[rami-a]

Acceptance Criteria

• When category is expanded the header moves to the top of the screen
• Header then has translucency and demos scroll beneath it
• This could maybe be done by presenting a new route with a custom transition (maybe using a Hero)
• The UX is a bit unclear as to what happens when you scroll to the top

Related to https://github.com/material-components/material-components-flutter-gallery/issues/346

Migrated from https://github.com/material-components/material-components-flutter-gallery/issues/100