[keyserver] Dedup notifTextsForMessageInfo calls in sendPushNotifs

Summary: We don't need to call this separately for each platform... the result is the same each time.

Test Plan: Flow, and tested in combination with the next diffs to confirm that getting @-tagged in a message that triggers the creation of a sidebar will update the SIDEBAR_CREATION notif to be suffixed with "and tagged you"

Reviewers: atul

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D6935