remoteMixStream.onUpdate event can not trigger when remoteStream count changed

[zhangyihui1]

1: user1 and user2 join room 2: user1 publish localstream to mcu 3: check user1 and user2 mixStream onUpdate event

expect: user1 and user2 mixStream should trigger onUpdate event actual: user1 and user2 mixStream can not trigger onUpdate event