setActiveTrack and setBufferProvider can't fail

Return void, not status_t, from setActiveTrack and setBufferProvider.

These methods returned status_t, but the callers never checked the
return value.  Since these aren't externally visible APIs, they now
return void, and assert on bad input parameters.

Change-Id: I530ed29484596ae41e8659826ca425149c51c2a1
diff --git a/services/audioflinger/AudioMixer.h b/services/audioflinger/AudioMixer.h
index 75c9170..6f31c5d 100644
--- a/services/audioflinger/AudioMixer.h
+++ b/services/audioflinger/AudioMixer.h
@@ -81,10 +81,10 @@
     status_t    enable(int name);
     status_t    disable(int name);
 
-    status_t    setActiveTrack(int track);
+    void        setActiveTrack(int track);
     status_t    setParameter(int target, int name, void *value);
 
-    status_t    setBufferProvider(AudioBufferProvider* bufferProvider);
+    void        setBufferProvider(AudioBufferProvider* bufferProvider);
     void        process();
 
     uint32_t    trackNames() const { return mTrackNames; }