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