Introduce a way to turn a fullscreen window into freeform

Change the exitFreeformMode to toggleFreeformMode. Now the original
maximize button will work as maximize/restore button.

Test: go/wm-smoke
Test: Manual test.
Bug: 117075771
Bug: 124475503

Change-Id: I268d053ed0e665b8ac5e56019c6ae8f8b7ee1317