libbinder: add makeDefaultRpcTransportCtxFactory

Add a new OS-specific function that creates a new
instance of the default RpcTransportCtxFactory.
This is needed because Android and Trusty have
different default transports: RpcTransportRaw and
RpcTransportTipcTrusty, respectively.

Bug: 230135749
Test: presubmit
Change-Id: I4abd443fe9a08c1fa0cc41dfca7ef1cdb69fe0fb