libbinder: RpcConnection: add ID

In preparation to have the server be able to distinguish clients and
clients to be able to dynamically create threads that are assigned to
them.

Future considerations:
- make ID impossible to guess (right now, one client might be able to
  get ahold of a thread from a server). We may implement something here
  or go for something existing like TLS.
- combine getting max threads and this? will wait until dynamic threads
  are actually implemented and we know we need this ID and we're looking
  at performance. For now this is a placeholder to enable dynamic client
  APIs.

Bug: 185167543
Test: binderRpcTest
Change-Id: If8563c69930c23b9ca91090b4f59ef1f51073f24