Keep the host audio threshold API compatible with Android signatures

tools/chatgui-android/video-tests/src/main/kotlin/android/media/HostStreamAudio.kt

@@ -41,12 +41,12 @@ class AudioTrack {
     var playState = 0
         private set
     val bufferSizeInFrames = 960
-    var startThresholdInFrames = 1
-        private set
+    private var threshold = 1
+    val startThresholdInFrames get() = threshold
     var playbackHeadPosition = 0
         private set
     val underrunCount = 0
-    fun setStartThresholdInFrames(value: Int) { check("threshold" !in failures); startThresholdInFrames = value }
+    fun setStartThresholdInFrames(value: Int) { check("threshold" !in failures); threshold = value }
     fun write(pcm: ShortArray, offset: Int, count: Int, mode: Int): Int {
         check(!released); check("write" !in failures); playbackHeadPosition += count / 2; return count
     }