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/media/libnbaio/NBLog.cpp b/media/libnbaio/NBLog.cpp
index 4d14904..c728e3e 100644
--- a/media/libnbaio/NBLog.cpp
+++ b/media/libnbaio/NBLog.cpp
@@ -67,6 +67,7 @@
 /*static*/
 size_t NBLog::Timeline::sharedSize(size_t size)
 {
+    // TODO fifo now supports non-power-of-2 buffer sizes, so could remove the roundup
     return sizeof(Shared) + roundup(size);
 }