[CommCoreModule] Make GRPCStreamHostObj.readyState readonly

Summary: Shouldn't be able to set readyState from the JS side

Test Plan: NA, still builds as expected

Reviewers: ashoat, varun, karol-bisztyga, palys-swm, def-au1t, jimpo

Reviewed By: ashoat

Subscribers: Adrian, benschac

Differential Revision: https://phabricator.ashoat.com/D3128