When Gemini Live was configured with local VAD (server-side VAD disabled),
the service was listening for the wrong frame types and not sending
ActivityStart/ActivityEnd events to the server. Now it listens for
VADUserStartedSpeakingFrame/VADUserStoppedSpeakingFrame and sends the
appropriate activity signals when local VAD is in use.
Also removes the unnecessary local SileroVADAnalyzer from server-side VAD
examples and adds a new 26a example demonstrating local VAD configuration.
These messages are developer instructions to the assistant (e.g. "Please
introduce yourself to the user"), not simulated user input. The
"developer" role is semantically correct for this purpose.
