[CommCoreModule] Introduce PlatformSpecificTools::getDeviceOS()

Summary: We need deviceOS for SessionSignature(...) unary gRPC call

Test Plan: Call function and log result. Only tested on iOS thus far, will test android before landing

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

Reviewed By: ashoat

Subscribers: Adrian, benschac

Differential Revision: https://phabricator.ashoat.com/D3153