Effect for multichannel PCM downmix to stereo

First pass at implementing an audio effect whose role is to
 downmix multichannel PCM buffers to stereo.
The effect is not handling volume changes.
The effect code here handles quad, 4.0, 5.1 and 7.1 input configurations,
 to optimize the most commom configurations, and does not yet handle
 generic multichanel configurations.

Change-Id: I74d04bd961348f3f0e4ae7714b70e620808a0829

Author: jmtrivi

media/libeffects/downmix/Android.mk

@@ -0,0 +1,28 @@
+LOCAL_PATH:= $(call my-dir)
+
+# Multichannel downmix effect library
+include $(CLEAR_VARS)
+
+LOCAL_SRC_FILES:= \
+	EffectDownmix.c
+
+LOCAL_SHARED_LIBRARIES := \
+	libcutils
+
+LOCAL_MODULE:= libdownmix
+
+LOCAL_MODULE_TAGS := optional
+
+LOCAL_MODULE_PATH := $(TARGET_OUT_SHARED_LIBRARIES)/soundfx
+
+ifeq ($(TARGET_OS)-$(TARGET_SIMULATOR),linux-true)
+LOCAL_LDLIBS += -ldl
+endif
+
+LOCAL_C_INCLUDES := \
+	system/media/audio_effects/include \
+	system/media/audio_utils/include
+
+LOCAL_PRELINK_MODULE := false
+
+include $(BUILD_SHARED_LIBRARY)