audio: utils: Add flush() and size() methods to MonoPipe

Add two new methods to the MonoPipe class:

- flush(): Flushes the audio frames available in the pipe.
  It should be called when te pipe is already in shutdown
  state to ensure that no more new data is pushed.

- size(): Queries the pipe size.

Change-Id: Ia8c2ac287a475bd3e72f48368d297fc544b19eed
Signed-off-by: Misael Lopez Cruz <misael.lopez@ti.com>
2 files changed