Fix PIP can't touch when back to home

When findPositionForStack, consider only 2 stack exist.
One is pip and another is the stack itself.
for case that has always-on-top window exist,
max should be 0 instead of topChildPosition.

Bug: 69370884, 111581878
Test: go/wm-smoke
Test: atest DisplayContentTests
Test: atest WindowContainerTests
Change-Id: Ia36290e98263027231491d66dbe953eccb6c925c