[lib] Return EntityText from getMessageTitle

Summary:
We need to resolve this EntityText at callsites, so we'll return it directly.

Initially the callsites will call entityTextToRawString same as before, but in following diffs I'll refactor it.

Depends on D6561

Test Plan: Tested in combination with following diffs, where the message title is actually used. Also Flow

Reviewers: atul

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D6562