libbinder: limit RpcTransport visibility

I always wanted to restrict the visibility of this
class, but it proved difficult in the original RPC
transport implementation. Now that we are considering
adding more transports, I'm adding an explicit list of
transports here. The reasoning is in the code.

Bug: N/A
Test: compile
Change-Id: Ib841e6c1c7cb6b59a6ca3aa15bbd94e66be3f6e8