Rename audio_utils/conversion.h to mono_blend.h

There is only one function declared in that header file, and it is
mono_blend, so a more specific filename may be easier to understand.

Test: compiles OK
Change-Id: Ie2aec6c706b1822581b34d3702818b8200a1db8a
diff --git a/services/audioflinger/Threads.cpp b/services/audioflinger/Threads.cpp
index b10e42c..b3134eb 100644
--- a/services/audioflinger/Threads.cpp
+++ b/services/audioflinger/Threads.cpp
@@ -36,7 +36,7 @@
 
 #include <private/media/AudioTrackShared.h>
 #include <private/android_filesystem_config.h>
-#include <audio_utils/conversion.h>
+#include <audio_utils/mono_blend.h>
 #include <audio_utils/primitives.h>
 #include <audio_utils/format.h>
 #include <audio_utils/minifloat.h>