Cache last valid drag area to check if task needs to be repositioned on

display change

Previously, we checked if the task was freeform and, if so, we
calculated draggable area to see if the task needs to be repositioned
when the display is rotated.

This logic is bad because we are assuming we will get a valid drag area
if the task is in freeform (aka we have an app header) which is now not
true since the caption controller is released when the task is not
visible and should not be assumed in the first place.

The valid drag area will now be cached in the windowDecor, and, the task
will check if repositioning is needed only if the decor returns a valid
drag area.

Bug: 441287483
Test: Manual testing
Flag: EXEMPT minor bugfix
Change-Id: Icb82e53927e336ef79b783660b77a39b908eea85