[Media] Card carousel, top-level view-model, and interactor interface.

- Card carousel - the last, top-level composable for the UI element;
  including the Media(...) single top-level composable API
- Top-level view-model - MediaViewModel is the single Activatable
  view-model of the entire system
- Interactor interface - MediaInteractor defines the contract between
  the new UI and the rest of the system

Bug: 397989775
Test: tested in testbed app with fake data, see b/397989775 #comment4 for a screen recording of the entire thing in action in the compose gallery app
Test: once we have a real implementation of MediaInteractor, it's
worthwhile to add an integration unit test that uses the view-model, the
real interface, and a fake data at the top of the upstream of the stack
Flag: EXEMPT - the code isn't used anywhere yet

Change-Id: I6c474693e83c796914e81c49c628a4bb6d2c1258