Use zero relative position for activity animation

If the activity doesn't fill task (e.g. max aspect ratio), there
will have letterbox surfaces to fill the remaining area. Since
the letterbox surfaces animate together with activity, the relative
position to task should be always zero.

Fixes: 154886867
Test: atest AppWindowTokenTests#testTransitionAnimationBounds

Change-Id: I9dabb3124ac163aec95d21849a859251bad7e4bd