Conversation controls should no-op or expose capability state when disconnected

[kalvin807]

Why

1. App code had to wrap sendUserActivity() behind status === "connected" checks.
2. setMicMuted is failing in text-only conversations. App code had to check if a conversation is going to be voice or text before starting a conversation

What

1. make calls like sendUserActivity() safely no-op when no active conversation exists
2. make micMuted and setMuted() should be harmless in text-only mode