mako: enable echo cancellation for voice communication

This greatly improves the quality of VoIP calls, tested on the app
"LINE". Without this, the other party will be able to hear echo
of their own voice.

Reference: http://source.android.com/devices/audio_implement.html#preprocessing

Change-Id: Id0ccb732778ecc8d4c9e140249ec230654f351cc

audio_effects.conf

@@ -148,3 +148,9 @@ effects {
 #
 # TODO: add default audio pre processor configurations after debug and tuning phase
 #
+
+pre_processing {
+  voice_communication {
+    aec {}
+  }
+}