[WM Shell] Move `isDesktopModeShowing` to DesktopTasksController since it is only used there.

This is part of the refactor to simplify the structure and communication between `DesktopModeTaskRepository` and `DesktopTasksController`.

In future CLs, the active + visible tasks will be completely removed so consolidate the usage for that behavior. This CL is a no-op with no change in behavior.

Change-Id: I706875903a5a560b1113f7287f3f2ee7b1b5ad5f
Bug: 332682201
Flag: EXEMPT (cosmetic changes only)
Test: atest DesktopTasksControllerTest
Test: atest DesktopModeTaskRepositoryTest