Always check front stack on its current display

Previously checks for front stacks were looking among
stack on primary display only. This doesn't make any
sense for stacks on secondary screens, so let's always
check among other stacks on its current display.

Bug: 34862802
Test: Manual.
Change-Id: I29ee160d455719d75f4a5981a6ba9c60f3d90084
3 files changed