Audio Effect API: process reverse stream function

Added function to audio effect interface for processing of
a reverse stream.
This is necessary for audio pre processes like echo cancellation.

Change-Id: I6e12d79dbbed6376acdfc79304b8c0ab3f705eae
diff --git a/media/libeffects/testlibs/EffectEqualizer.cpp b/media/libeffects/testlibs/EffectEqualizer.cpp
index c2ffce5..b22ebec 100644
--- a/media/libeffects/testlibs/EffectEqualizer.cpp
+++ b/media/libeffects/testlibs/EffectEqualizer.cpp
@@ -736,7 +736,8 @@
 const struct effect_interface_s gEqualizerInterface = {
         Equalizer_process,
         Equalizer_command,
-        Equalizer_getDescriptor
+        Equalizer_getDescriptor,
+        NULL
 };