NBAIO: re-implement NBAIO Pipe and MonoPipe using fifo

Also removed const from NBAIO_Sink::availableToWrite() because
at least one implementation can no longer implement the const-ness.

Test: normal mixer, tee sink, and remote submix still work
Change-Id: I8461177efdf53bba8295b147e97835b018804903
diff --git a/include/media/nbaio/PipeReader.h b/include/media/nbaio/PipeReader.h
index 00c2b3c..70ecb34 100644
--- a/include/media/nbaio/PipeReader.h
+++ b/include/media/nbaio/PipeReader.h
@@ -57,7 +57,7 @@
 
 private:
     Pipe&       mPipe;
-    int32_t     mFront;         // follows behind mPipe.mRear
+    audio_utils_fifo_reader mFifoReader;
     int64_t     mFramesOverrun;
     int64_t     mOverruns;
 };