Factor out buffer provider code from AudioMixer

In preparation for playback rate support and timestretching.

Bug: 19196501
Change-Id: I435accb852d32110dd0b3a9917488522c567ba80
diff --git a/services/audioflinger/tests/Android.mk b/services/audioflinger/tests/Android.mk
index 8604ef5..76997be 100644
--- a/services/audioflinger/tests/Android.mk
+++ b/services/audioflinger/tests/Android.mk
@@ -39,6 +39,7 @@
 LOCAL_SRC_FILES:= \
 	test-mixer.cpp \
 	../AudioMixer.cpp.arm \
+	../BufferProviders.cpp
 
 LOCAL_C_INCLUDES := \
 	$(call include-path-for, audio-effects) \