Use surface position for starting reveal animation

The surface position is basically relative frame - surface insets.

This aligns the same behavior as WindowState#startAnimation(anim)
that uses mSurfacePosition to animate.

If the app sets surface insets, its content will draw from the offset,
e.g. (50,50), then the surface position will be (-50,-50) to make
the window show at the requested position.

Bug: 328282706
Test: Cold or warm launch an app with calling
      getWindow().getDecorView().setElevation(100f);
      in onCreate. The activity window should not have a weird
      offset when playing starting window reveal animation.
Change-Id: I8c4f79d513014997f28c915682640c27d761a26c