Use only the icon size for surface to draw the icon into

- The original change ag/16081867 repourposed OutlineManager which
  actually drew in the outline around each split task, but the
  SplitDecorManager only needs to draw the icon (the background is
  on another surface).  This will also be obsolete once we migrate
  to/merge SplitDecorManager and ResizeVeil in the future.

Bug: 335975211
Test: Take a perfetto trace and verify that the requested surface
      bounds correspond to the desired icon size

Change-Id: Ib4fa8921ea114b32e54dc35a09e86bab6d838420