Ensure isTopBucket() gets set when there is just one notification

My descendants will vilify this CL for generations to come. We'll
clean it up for R, but this is our last, best hope for fixing things
in Q.

Bug: 138775282

Test: manual
Change-Id: I615b2f7fddca30dae67dbaab0e5d54a824a4c441
(cherry picked from commit 2d35980e)