audio: Re-worked sound pre- and post-processing

In order to activate proprietary sound pre- and post-processing,
the following parts of audio HAL were revised:

* Activated ENABLE_PREPROCESSING directive in Android.mk
* Removed unused variables and functions
* Re-written set_preprocessor_echo_delay() function

Change-Id: I868087f422764895df7d2bc6bbf4bbd76672b8e7
diff --git a/audio/Android.mk b/audio/Android.mk
index f369f7c..64ada48 100644
--- a/audio/Android.mk
+++ b/audio/Android.mk
@@ -51,7 +51,7 @@
 	$(call include-path-for, audio-effects)
 
 LOCAL_CFLAGS := -Werror -Wall
-#LOCAL_CFLAGS += -DPREPROCESSING_ENABLED
+LOCAL_CFLAGS += -DPREPROCESSING_ENABLED
 
 LOCAL_MODULE := audio.primary.$(TARGET_BOOTLOADER_BOARD_NAME)