Updating tiling handle color to be the theme outline variant.

This CL updates the tiling divider color to be the theme outline variant
and updates the divider color on theme change.

Flag: com.android.window.flags.enable_tile_resizing

Test: device testing of handle color changes, just UI changes
so no unit tests

Bug: 402779110
Change-Id: Ifb4a173201ebf95f1b9d61a395585d1ba830863d