Stabilize channel state API

ManagedChannel.getState(), ManagedChannel.notifyWhenStateChanged(), and ConnectivityState.

Start with ManagedChannel.getState(), ManagedChannel.notifyWhenStateChanged(), and ConnectivityState, then review their current contracts and usages. The issue does not identify files, tests, or a concrete change, so the intended stable behavior and completion criteria must be established before implementation.

索引モデルが issue の本文から書いたものです。