Refactor: Extract GetThumbnailPositionUseCase from TTVM

This CL deletes TaskThumbnailViewModel and refactored ThumbnailPosition
to be propagated from the TaskView into TaskContainer.

Fix: 396147816
Doc: go/launcher-overview-unified-taskviewmodel
Flag: com.android.launcher3.enable_refactor_task_thumbnail
Test: OverviewImageTest
Test: TaskViewModelTest
Test: GetThumbnailPositionUseCaseTest
Test: GetThumbnailUseCaseTest
Change-Id: I9cc68a8007835f0dc4846b9bf71377c81d19b27b