Add RpcServer::setRootObjectWeak(wp)

If the root binder object also holds a strong reference
to the RpcServer, to avoid circular dependency, the RpcServer
must hold a weak pointer of the RpcServer instead.

Test: binderRpcTest
Change-Id: I0d0c1f96a6a6d5339e2699f38edfaeb7ec2fd7f9