Fixed logic issue with WC.supportsSplitScreenWindowingMode()

A container can support split-screen windowing mode while in pinned or
freeform mode. However, there were some call-points that were using the
method to determine if the container is affected by the split-screen
stack resizing. So, just created a new method
ActivityStack.affectedBySplitScreenResize() for those.

Change-Id: Iae56bcd2cb696179dda39b77a69035d5f067a23d
Fixes: 68762925
Test: go/wm-smoke
Test: Manual steps from bug.