Hide taskbar content while dreaming.

When entering the dreaming state, the TaskbarDragLayer is faded out.
Upon wake up, a slight delay is added to  allow the SFPS reader to do
its magic, so the lockscreen-navbar does not pop-in just to be removed
again.

Bug: 271440683
Bug: 275319714
Fixes: 271440683
Test: manual (http://shortn/_cQudGXDSDU)
Change-Id: I34e02f02288bace39626d531d115fc994b11f371