Track visible tasks in DesktopModeController and add a listener

- Adds DesktopModeTaskRepository to FreeformTaskListener to track
  visible freeform tasks
- Adds a listener on DesktopModeController to allow sysui to find
  out about freeform tasks becoming visible
- Adds a sysui state flag for freeform tasks being visible

Bug: 245569277
Test: manual - add freeform task => launcher contents are gone
             - remove freeform task => launcher contents are visible
Test: WMShellTest DesktopModeTaskRepositoryTest
Change-Id: I2d717052678bc35a4c59b0bc03e0c4d22e6b103d