add background dispatcher

Author: VinayGuthal

firebase-vertexai/src/main/kotlin/com/google/firebase/vertexai/type/LiveSession.kt

@@ -208,7 +208,7 @@ internal constructor(
     audioHelper = AudioHelper()
     audioHelper!!.setupAudioTrack()
     fillRecordedAudioQueue()
-    CoroutineScope(Dispatchers.Default).launch { sendAudioDataToServer() }
+    CoroutineScope(backgroundDispatcher).launch { sendAudioDataToServer() }
     fillServerResponseAudioQueue()
     playServerResponseAudio()
     delay(1000)