libbinder: RPC explicit connect thread ownership

- thread is detached when it is no longer owned (avoids abort)
- RpcServer passes connection thread ownership to RpcSession before it
  lets go of its lock (otherwise, it's possible to take the lock for
  both the session and the server, and have a relevant thread which
  isn't reflected as owned in either of these objects). Currently this
  only affects the fuzzer, but it will also be important for shutting
  down these threadpools.

Future considerations - this code has a few messy parts, but it will
have to be rewritten to avoid the std::thread constructor (which throws
exceptions) and also to read a header instead of an ID.

Bug: 185167543
Test: binderRpcTest, binder_rpc_fuzzer (which is in-progress)
Change-Id: Ide630e36595d09a88e904af2e9ab6886ae4f2118