grpclb: LoadBalancer.requestConnection() may be ignored if it's called before subchannels are created

### What version of gRPC-Java are you using?
v1.32.1

### What did you expect to see?
Calling `ManagedChannel.getState(true)` will make the channel to try to connect to the server at least once, no matter when it's called.

### What did you see instead?
Calling `ManagedChannel.getState(true)` makes the channel try to connect to server only if it's called after subchannels are created, i.e., after name resolution in pick-first, and after backend addresses are received in grpclb. If `getState(true)` is called before that point, the channel won't try to connect to servers after subchannels are created, unless `getState(true)` is called again.

/cc @ejona86

Start with ManagedChannel.getState(true) and LoadBalancer.requestConnection(), then compare the pick-first and grpclb timing described in the report. Reproduce a call made before subchannels are created; the fix is complete when that call still causes a server connection after subchannels become available.

索引モデルが issue の本文から書いたものです。