[native] Clean up ComposedMessage

Summary:
Two minor fixes here for issues introduced in D7067:

1. Rename messageBoxStyleContainerStyle to messageBoxContainerStyle (typo)
2. Avoid inline styles in favor of using combined style sheet (see leftChatContainer and rightChatContainer)

Test Plan: Flow

Reviewers: ginsu, atul

Reviewed By: ginsu

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7696