Update listeners of stashed state after add

When a VisibleTasksListener is added to DesktopModeTaskRepository, send
an update with the current stashed value.
This is in addition to the update on visibility.

Launcher will start using the visibility listener for updating its
state. Remove the TODO for it.

Test: atest DesktopModeTaskRepositoryTest
Bug: 280490493
Change-Id: I9d6638a73283f5121599384b80ea84f671bb884b