libbinder: RPC socket bound to service thread

This is in preparation of transforming binder RPC to have a more
traditional server achitecture where the server accepts new connections,
and arbitrarily many can be added.

Before this change, there were a few issues:
- threads might call accept, but not use the FD they accepted (threads
would accidentally trade FDs, and this could create confusing logs)
- if a server thread disconnected or finished, another thread might try
to reuse the FD being used there

Now:
- server sockets are associated with the thread from the beginning
- when a connection disconnects, the server also removes the connection

Bug: 185167543
Test: binderRpcTest (note added ~RpcConnection assert)

Change-Id: Icf50a23ba52167c6fbe04ad1a77cfcb6fb3fcc9c