hal: Reduce minimum offload fragment size for PCM offload

- Video frame drops are observed on RTSP streaming
  for low sampling rate clips

- For PCM offload audio track uses server proxy position
  value to caluclate render position.
  Due to high minimum fragment size value of PCM offload,
  low sampling rate clips are using bigger audio buffer
  than desired 20ms PCM offload buffer. This results
  improper audio render position return to Nuplayer,
  which leads to video frame drops

- Reduce minimum fragement size for PCM offload from 4k
  to 512

Change-Id: I661f1ffe7cfd41edead82d06063802a3ad7b9b66