make libaudioflinger symbols visibility hidden

we export only symbols needed by clients of this library.
this saves about 130KB (1/3rd of the lib size)

Change-Id: Id81f3ecb299ee3abc0811915cf6efe87180bf15c
diff --git a/services/audioflinger/Android.mk b/services/audioflinger/Android.mk
index 061a079..714854e 100644
--- a/services/audioflinger/Android.mk
+++ b/services/audioflinger/Android.mk
@@ -81,6 +81,8 @@
     LOCAL_CFLAGS += -DANDROID_SMP=0
 endif
 
+LOCAL_CFLAGS += -fvisibility=hidden
+
 include $(BUILD_SHARED_LIBRARY)
 
 #