Allow back gesture on bouncer/shade over dream

Remove back gesture restriction when the bouncer or notification shade
is showing over the dream, to allow the user to swipe back to close the
bouncer/shade.

Fixes: 362841645
Test: atest DreamOverlayServiceTest
Flag: EXEMPT bugfix
Change-Id: Idaf4cc0567cde6226712ca45e97597927f1abd10