Ports WebRTC's rnn_vad (as TS3 embeds it) to Java: LPC, pitch
estimation, spectral features and the RNN itself, feeding a
speech-probability detector that replaces the old SpeechDetector.
Also switches the volume-gate threshold from raw dBFS to
InputLevel's scale, matching TS3's own slider and range, with a
migration for settings saved under the old key.
