Add VAD demo for Java API (#928)

sherpa-onnx/java-api/src/com/k2fsa/sherpa/onnx/WaveWriter.java

@@ -0,0 +1,15 @@
+// Copyright 2024 Xiaomi Corporation
+
+package com.k2fsa.sherpa.onnx;
+
+public class WaveWriter {
+    public WaveWriter() {
+    }
+
+    public static boolean write(String filename, float[] samples, int sampleRate) {
+        WaveWriter w = new WaveWriter();
+        return w.writeWaveToFile(filename, samples, sampleRate);
+    }
+
+    private native boolean writeWaveToFile(String filename, float[] samples, int sampleRate);
+}