xds: Lazily initializing child pickfirst LBs is not happening when TF is reported at the point that no other children are connecting.

[larry-safran]

There are two conditions for lazily initializing child pickfirst LBs mentioned in A61:

1. When the endpoint chosen by the ring is in TF. This is present in Java.
2. When a child pickfirst balancer reports TF: If no other children are connecting, we initialize the first child which is IDLE. This happens outside the picker and is missing in Java.

This was working prior to #10610

[zihenzzz]

If it's a bug, I can try to fix it