[native] style emoji keyboard

Summary:
Synced with @ted about how the emoji keyboard should look and he gave me feedback to decrease the border radius and remove the border width from the search bar of the emoji keyboard. The next diff will address changing the colors based on the app theme.

Here are some docs I found that helped me implement the styling of the emoji keyboard:

https://thewidlarzgroup.github.io/rn-emoji-keyboard/docs/api/styles#searchbar

Depends on D8288

Test Plan:
Please see the screenshots below

Before:

After:

Reviewers: atul, kamil

Reviewed By: kamil

Subscribers: ashoat, tomek, ted

Differential Revision: https://phab.comm.dev/D8309