Fix VAD interface to support audio chunks of any length

The webrtcvad library expects frames of fixed size, but Genie will
pass the buffers as they come from nodejs streaming code without
any slicing. Do the slicing and concatenation of frames here so
we only call the library with frames of fixed size.

service/platform/index.js

@@ -254,6 +254,7 @@ try {
 class VAD {
     constructor() {
         this._instance = null;
+        this._previousChunk = null;
         this.frameSize = 0;
     }
 
@@ -275,10 +276,22 @@ class VAD {
     process(chunk) {
         if (!this._instance)
             return false;
-        let n = chunk.length % this.frameSize, r = 0;
-        for (let i = 0; i < n; i++)
-            r += this._instance.process(chunk.slice(i * this.frameSize, this.frameSize));
-        return r;
+
+        if (this._previousChunk)
+            chunk = Buffer.concat([this._previousChunk, chunk], this._previousChunk.length + chunk.length);
+
+        let anySound = false;
+        let offset;
+        for (offset = 0; offset < chunk.length && offset + this.frameSize <= chunk.length; offset += this.frameSize) {
+            const sliced = chunk.slice(offset, offset + this.frameSize);
+            const sound = this._instance.process(sliced);
+            anySound = anySound || sound;
+        }
+        if (offset < chunk.length)
+            this._previousChunk = chunk.slice(offset);
+        else
+            this._previousChunk = null;
+        return anySound;
     }
 }