Move active task tracking to a repository

We were tracking active freeform tasks in the RecentTasksController.
This causes some issues when we want to share that information to other
controllers like DesktopModeController.
Move tracking of active tasks to a separate repository that controllers
can query and listen for updates.

Bug: 244348395
Test: atest DesktopModeTaskRepositoryTest RecentTasksControllerTest
Change-Id: I7a460020f6b2133225e292a04f29ae1dd27e964b