Two buffers are not queued

[MatthewLM]

Hi. I've not tried this project out but I was looking through it and saw that only one buffer is initially queued which would likely cause the audio to cut out whilst waiting for samples:

https://github.com/google/music-synthesizer-for-android/blob/master/app/src/main/jni/android_glue.cc#L175