Split audio effects headers

There can be different use cases with the code that imports
headers from audio_effects (each subsequent case is a more
specialized version of the previous one):

  1. The code only needs access to general structures
     like effect_descriptor_t:
     ==> include system/audio_effect.h
     Example: AudioSystem in libmedia

  2. The code that needs access to effect UUID and parameter
     enums:
     ==> include system/audio_effects/effect_foo.h
     Examples: media framework effects interfaces, libwilhelm

  3. The code that needs to call into HAL:
     ==> include hardware/audio_effect.h
     Examples: libaudiohal (implementation part) and libeffects

  4. The code that implements a particular effect:
     ==> include audio_effects/effect_foo.h
     Examples: cts/.../CTSTestEffect.cpp
               stuff in hardware/, vendor/

Note that AudioPolicy still deals with the effects HAL directly,
this needs to be fixed.

Change-Id: I963335ede6a196f9225caf21e1a01d03635e363c
Test: make