balancer: stop using picker from old balancer with switching balancers

Author: menghanl

Diff for: balancer_conn_wrappers.go

@@ -151,6 +151,7 @@ func (ccb *ccBalancerWrapper) watcher() {
 			for acbw := range scs {
 				ccb.cc.removeAddrConn(acbw.getAddrConn(), errConnDrain)
 			}
+			ccb.UpdateBalancerState(connectivity.TransientFailure, nil)
 			return
 		default:
 		}