Fix background threading for IOS

[sweiss435]

PeerConnection disconnects when IOS goes to background if I create peer connection on main scope. However, if I try and use GlobalScope, when I do:
val config = RtcConfiguration()
I get:
Trying to access top level value not marked as @ThreadLocal or @SharedImmutable from non-main thread
kotlin.native.IncorrectDereferenceException: Trying to access top level value not marked as @ThreadLocal or @SharedImmutable from non-main thread
I believe you need to add @ThreadImmutable like:
https://github.com/realm/realm-kotlin/pull/822/files
to values in classes like RtcConfiguration like
@ThreadImmutable
val js: Json