Suppress transient bars over dreams.

This changelist prevents transient bars over dream windows.
This allows for suppressing these UI elements when other swipe
affordances are provided by the SystemUI.

Bug: 267565290
Test: atest DisplayPolicyTests#testTransientBarsSuppressedOnDreams
Test: manual: swipe up and down from edges of device on a dream
      and observe now transient bars
Change-Id: I5be342ed21174ff6e2c2cf87cea6202773e5f51a