Fixing tiling divider rounded corners and spec colors

This CL updates the divider/handle/rounded corner colors for the tiling
divider to match the design spec.

It also updates the rounded corner radius from being device dependent to
constant radius based on the latest design decisions.

Flag: com.android.window.flags.enable_tile_resizing

Test: unit tests and on device testing

Bug: 379664865, 382445299
Change-Id: Icbd67406ab266918d58756bba3b6212fe340649d