Fix CTS failure for waterfall device

When entering to or exiting from Pip mode, the PipTaskOrganizer will
set the windowing mode of the activity of the task to
WINDOWING_MODE_FULLSCREEN and temporarily set the bounds of the task to
fullscreen size for transitioning.
It will get the wrong smallestScreenWidthDp if the calculation is based
on this temporary fullscreen bounds.

We should just inherit the value from parent for this temporary state.

Bug: 206871341
Test: atest PinnedStackTests

Change-Id: If3001b13b48b1e3124b9bd898ab740a2d3025df0