libbinder: RPC save notify on transferring thread

We don't need to notify RpcServer when thread ownership is moved from a
connection thread to a session thread since this can't affect shutdown.

Bug: 185167543
Test: binderRpcTest, run binder_rpc_fuzzer for a minute
Change-Id: I8077beabe8176470f95b17a1f93c3fb731fdf8ca