Update initial staged split UX.

- Introduce inset to the staged split view
- Update width of the staged split view
- Introduce rounded corners to staged split view. It's tricky to animate it in with smooth transition considering the rounded corners. The best way to handle it is to draw content as if part of it is off the screen, so that when it's animated in, the rounded corners can slide in onto the screen correctly.

Fixes: 219085340
Test: https://recall.googleplex.com/projects/f46cfe9c-8076-4efe-bf8a-b1cc4f1f5e1b/sessions/09a99b75-3614-4d20-b6d4-a78108f769d8

Change-Id: I24d90cc9e2695d822cb2de8b21e2d5519f2e344d