Shows how to use the MediaCodec and MediaMuxer to create an mp4 file from PCM audio

This shows how to:

1. Record raw PCM from the microphone via AudioRecord
2. Encode it to AAC via MediaCodec
3. Mux it into an mp4 container via MediaMuxer

For most use cases it is a lot easier to just use MediaRecorder. This is only useful if you have externally generated PCM audio. This is most commonly seen in wav files.