Implement BottomSheet max width and centered.

Implement the design for larger screens. (The bottom gap is not included in this ticker)
![screen-shot-2019-02-01-at-3 00 56-pm-1](https://user-images.githubusercontent.com/9772967/52447167-46eef200-2b30-11e9-925e-6ca71c32971a.png)