Partial Screen Sharing: Task Switcher - Domain Layer implementation

Implements the domain layer:
- Defines model/state
- Implements the interactor that is responsible for knowing whether a
  task switch has happened, in the context of task projection
- Implements fake repositories to be used in unit tests

Bug: 286201261
Test: TaskSwitchInteractorTest.kt
Change-Id: I46ff1214c97a52e63893bc728c2fa191b08b7c6e