Only restore to mLastNonFullscreenBounds if requested bounds unset

Whenever a task changes windowing mode to freeform (or undefined and
its parent is freeform), the last non-fullscreen bounds are restored.
This CL changes that so that the restore bounds are used only when
the requested override window config bounds are null/empty, to
prevent overwriting bounds set by wm shell in a transition's WCT.

Bug: 235885662
Test: atest TaskTests
Change-Id: Iba9c94d700ad4e1ef05073353c0d47c34daf5b26