Use the icon size of starting surface for split decor

Use a fixed icon size and fit to centor scale for split decor to avoid
unexpected icon size. Also sync the same icon size to drop zone area.

Bug: 228928242
Test: Manual resize the split and check the icon size.
      Manual drag to split and check the icon size.
Change-Id: Idaf523810e3913a22787925817b6013464818147