[lib] Use hook instead of connect function in MessageHandler

Test Plan: Checked if onMessage still fired and worked correcly on web and native

Reviewers: palys-swm, ashoat, atul

Reviewed By: ashoat

Subscribers: ashoat, KatPo, zrebcu411, Adrian, atul

Differential Revision: https://phabricator.ashoat.com/D828