Disable edge back gesture handling for dreams when overlay is active.

This changelist adds new functionality for disabling gesture handling on
specific activity components at any given time. DreamOverlayService
uses this new functionality to disable gestures on DreamActivities when
they start.

Fixes: 333734282
Test: atest DreamOverlayServiceTest#testDreamActivityGesturesBlockedOnStart
Test: atest DreamOverlayServiceTest#testDreamActivityGesturesUnblockedOnEnd
Test: atest GestureInteractorTest
Test: atest GestureRepositoryTest
Flag: EXEMPT bugfix
Change-Id: If67a2ba9e069455a9c21e8d934872459e7d1b9f1