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
Merged-In: I615b2f7fddca30dae67dbaab0e5d54a824a4c441
(cherry picked from commit 2d35980e)