binder: Add RpcTransport class.

RpcTransport represents the transport
layer. The transport layer may have security (TLS) enabled
or not, depending on how it is configured.

In libbinder, RpcTransportRaw implements RpcTransport without TLS.

Add RpcTransportShim that wraps usage of RpcTransport*. If TLS is not
requested, return RpcTranpsortRaw. Otherwise return RpcTransportTls.

RpcTransportTls will be added in follow up CL.

Bug: 190868302
Test: TH
Change-Id: Ic6c6bec4a2135e42f32840dcd5f60b57e57439b5