Make TasksRepository listen to changes in task visuals and update

To do this TaskVisualsChangedDelegate was added which multiplexes two listeners that are closely related.

Thumbnail overriding capabilities were also removed.

Fix: 342560598
Test: TaskVisualsChangedDelegateTest, TasksRepositoryTest
Flag: com.android.launcher3.enable_refactor_task_thumbnail
Change-Id: If065e4179cd1f15fe2cdf9b6bae51afcb57abcc6