Improve the staged split animation

- Introduce rounded corners (since we scale x and y differently, we can't use outline since it doesn't support rx and ry. It's achieved by custom drawing).
- Make sure the thumbnail content doesn't shift during the transition (we use custom cropping for TaskThumbnailView, and we have to do it accordingly here)

TODO: update UX of the initial split view (b/219085340)

Fixes: 194414938
Test: https://recall.googleplex.com/projects/f46cfe9c-8076-4efe-bf8a-b1cc4f1f5e1b/sessions/64953aa7-62ea-427c-8ec0-5f2bd96e4762
Change-Id: Id9a5d2f0f41cb4d619c8b3bd3a83c633e3d1f2de