Fix one-icon shelf flicker on lockscreen swipe-up

The flicker happens because floating point comparison
for viewStart and shelfStart is off by ~0.001f sometimes,
such that the notification is thought to be above the shelf,
causing the shelf to hide since it no longer has a full view.

Bug: 222123657
Test: NotificationShelfTest
Test: on lockscreen, swipe up with one icon in shelf
      => no flickering
Change-Id: Iad2bb24d4a51d5a831aa3193de10f768fa7c41be