Add fade in animation to tiling divider to make it look nicer upon tiling.

This CL adds a fade in animation of 300 ms, upon snap resizing left/right,
the divider won't appear instantly, but will start fading it after the
tiled app reaches it's destination.

Flag: com.android.window.flags.enable_tile_resizing

Test: on device testing of UI, really hard to unit test the UI here.

Bug: 379663061
Change-Id: I3cccd3f32ae019ba7e6cdbe9f1f08b95d8c22e00