Refactor DesktopModeTaskRepository to simplify and consolidate logic to use Transition Observers / Handlers. New logic will be added in a subsequent CL with flag-guarding (current changes are no-op).

Change-Id: I96467e8f93c8f8f7bed97073d8a4c78a8c4a6cb6
Bug: 332682201
Flag: EXEMPT (no-op for functionality)
Test: atest DesktopModeTaskRepositoryTest
5 files changed