Use switch in AudioMixer::setParameter

Replace series of if/then/elses by easier-to-read switch.  Also return
void instead of status_t, since callers weren't checking it. Assert on
bad input parameters.

Change-Id: Ie1f0a297977b28501d20e1af819afed9b4750616
diff --git a/services/audioflinger/AudioMixer.h b/services/audioflinger/AudioMixer.h
index f380b35..288c70e 100644
--- a/services/audioflinger/AudioMixer.h
+++ b/services/audioflinger/AudioMixer.h
@@ -81,7 +81,7 @@
     void        disable();
 
     void        setActiveTrack(int track);
-    status_t    setParameter(int target, int name, void *value);
+    void        setParameter(int target, int name, void *value);
 
     void        setBufferProvider(AudioBufferProvider* bufferProvider);
     void        process();