Calculate current position for window animation leash

In case mSurfacePosition hasn't be updated yet. This aligns
the same logic as startMoveAnimation.

Bug: 330721336
Test: Repeatedly showing a dialog. The position of dialog should
      not start from zero when animating.
Change-Id: I8a2ad5819a1a17b9da6d31b21d5955b5ef9e38d3