Android: implement ADM SetMicrophoneMute and allow opting out of stop-on-mute (parity with iOS)

[max-buster]

Summary

Since #206 (624fa1d), WebRtcVoiceSendChannel::MuteStream stops the ADM's recording when all send streams are muted, unless the ADM overrides IsStopOnMuteModeEnabled() to false. The iOS/macOS AudioEngineDevice opts out and mutes internally while keeping capture alive; the Android ADM has no override, so muting an audio track now fully releases AudioRecord.

Per the guidance in #246 — platforms that properly implement the ADM's Mute API can opt out individually — this issue proposes doing exactly that for Android.

Problem

For push-to-talk style apps, re-enabling the mic is a cold start: a new AudioRecord, AGC reconvergence, and on USB audio devices a full interface renegotiation (alternate setting selection, bandwidth reservation, sample rate setup). Speech during that window is lost, heard as a clipped first word on every transmission. Before #206, muting kept capture warm on Android.

Android already has a working warm-mute implementation — WebRtcAudioRecord.setMicrophoneMute() zero-fills the capture buffer on the record thread (sdk/android/src/java/org/webrtc/audio/WebRtcAudioRecord.java) — but it is only reachable from Java. The C++ AndroidAudioDeviceModule::SetMicrophoneMute() returns -1 "Not implemented" (sdk/android/src/jni/audio_device/audio_device_module.cc), so the fallback path in MuteStream is a no-op on Android, and there is no way to opt out of stop-on-mute.

Proposal

Keep stop-on-mute as the Android default; make the mute API real and the opt-out possible:

1. Implement SetMicrophoneMute on the Android ADM: bridge AndroidAudioDeviceModule::SetMicrophoneMute() through AudioRecordJni to the existing WebRtcAudioRecord.setMicrophoneMute(), so the non-stop mute path zeroes audio at the source — the same "mute inside the engine, capture stays alive" semantics as the iOS AudioEngineDevice.
2. Builder opt-out: add JavaAudioDeviceModule.Builder.setStopRecordingOnMute(boolean), default true (current behavior unchanged), plumbed through nativeCreateAudioDeviceModule into AndroidAudioDeviceModule, which overrides IsStopOnMuteModeEnabled() accordingly.

This mirrors the existing useStereoInput plumbing and is fully backward compatible. We understand the trade-off the default protects (OS mic-in-use indicator turns off while muted); apps that opt out accept the indicator staying on in exchange for warm capture.

Happy to submit a PR for this if the approach sounds acceptable.

Context

• Regression surfaced in LiveKit Android (io.github.webrtc-sdk:android builds containing 624fa1d, i.e. 137.7151.05+) for a PTT app using USB handset microphones. LiveKit Android v2.23.0 (webrtc 137.7151.04) is the last unaffected release; v2.23.1 (137.7151.05) the first affected.
• iOS had the equivalent problem (livekit/client-sdk-swift#422), resolved by the AudioEngineDevice opt-out in AVAudioEngine version AudioDeviceModule #158/Change default to stop recording when muted #206.
• Current workaround: publish the mic once and gate PTT with the ADM-level setMicrophoneMute from the app, which forfeits track-level mute semantics (server mute state, remote TrackMuted events) and transmits encoded silence for the call duration.