[native] Use hook instead of connect function and HOC in ComposeSubthreadModal.

Summary: Switched to hooks instead of the connect function in the ComposeSubthreadModal class.

Test Plan: Tested creating subthreads for open and secret. And tested to made sure messages and other features still work.

Reviewers: ashoat, palys-swm

Reviewed By: ashoat

Subscribers: KatPo, zrebcu411, Adrian, atul, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D732