Currently, TaskBar expansion is allowed when an activity context is
available. Since Dreams are implemented as an activity, this leads
to being able to expand the TaskBar over a dream. This changelist
updates the logic to not allow system gestures when dreaming.
Test: manual
Bug: 265755700
Change-Id: If58608c1e491da7906a2615681da0514fa49cfc8
Merged-In: If58608c1e491da7906a2615681da0514fa49cfc8