[native] Fix inline sidebar background color

Summary:
Fixes ENG-1954.
It turns out that sidebar wrapper/holder (with 100% width) had background color set to black. And because it has border radius set, the top-left corner looked OK (or the top-right in case when viewer is author of the message).

Fixed it by removing the background color and making the view opaque.

Test Plan:
Tested in iOS simulator, used RN Element Inspector.

Temporarily set the background to green and disabled cornerRadius (1st screen) to show the element. The last screen shows final effect

Reviewers: ashoat, tomek, atul

Reviewed By: atul

Subscribers: atul, abosh

Differential Revision: https://phab.comm.dev/D5325