Use same margins for widgets bottom sheet as all apps.

* Per UX discussion with jane and david, the full width looks too large
for the app info -> long press -> widgets bottom sheet.
* Earlier during POR, we decided to match them all, but after trying it out, we think smaller is better for the app specific bottom sheet.

* http://screencast/cast/NTg1Njk1MDUxOTU5NTAwOHxiMDU0YWJhMi04MQ
* http://screencast/cast/NjU0NjQyODgwMDI3MDMzNnw4NDNhMGVjMy0yMg


Bug: 328812171
Test: screencast
Flag: N/A
Change-Id: I9b0a996f80bac2639bcadf85480468ac886c7093