Add an optimized FIFO writer for 32-bit words

Also improve the FIFO comments

Test: not yet tested
Change-Id: I29b645c05800491661ec81e816a38544b64fcc90
diff --git a/audio_utils/Android.bp b/audio_utils/Android.bp
index 31d25df..c6999e5 100644
--- a/audio_utils/Android.bp
+++ b/audio_utils/Android.bp
@@ -22,6 +22,7 @@
         "ErrorLog.cpp",
         "fifo.cpp",
         "fifo_index.cpp",
+        "fifo_writer32.cpp",
         "format.c",
         "limiter.c",
         "minifloat.c",