Prevent touch event dispatch on the top edge when collapsed

For some reason, touch events at y=0 can get dispatched all the
way to Quick Settings, even though it is not visible. This change
prevents that from happening in the most narrow way I could find.

Fixes: 235889526
Test: atest and manual
Change-Id: Iedbb3b55a94864d320b886ba7c29913ad4b8b145