[native] Use getMessagePreview in native MessagePreview

Summary:
After D6357 updated getMessagePreview to return a style for each portion, we can update native to use that function to unify codepaths with web.

This should be a no-op. Compare to web's version of MessagePreview in D6358 – very similar code.

Depends on D6359

Test Plan:

before	after

(should be identical)

Reviewers: inka, ginsu, atul, tomek

Reviewed By: atul

Differential Revision: https://phab.comm.dev/D6360